Guide for authors

Prace przyjmowane są tylko przez system ScholarOne Manuscripts

General information and journal policy

Acta Haematologica Polonica (AHP) is the official journal of the Polish Society for Hematology and Blood Transfusion  and Institute of Hematology and Blood Transfusion. All articles published in Acta Haematologica Polonica are fully citable with a digital object identifier (DOI).

Your Paper Your Way

We now differentiate between the requirements for new and revised submissions. You may choose to submit your manuscript as a single Word file to be used in the refereeing process. Only when your paper is at the revision stage, will you be requested to put your paper in to a 'correct format' for acceptance and provide the items required for the publication of your article. To find out more, please go to the Preparation section below.

Criteria for manuscripts

The Editorial Board of Acta Haematologica Polonica takes under consideration for publication original articles with the understanding that neither the manuscript nor any part of its essential substance, tables or figures have been published previously in print form or electronically and are not under consideration by any other publication or electronic medium. This restriction does not apply to abstracts or press reports published in connection with scientific meetings. Copies of any closely related manuscripts should be submitted to the Editor along with the manuscript that is to be considered by AHP. AHP discourages the submission of more than one article dealing with related aspects of the same study. Each submission packet should include the statement signed by the first author that the work has not been published previously or submitted elsewhere for review and a copyright transfer.

Categories of articles

Accepted papers are published in the following journal sections: Original Research Article, Review Article, Editorial, Expert Guidelines, Short Communication, Letter to Editor, and Clinical Vignette. From 2020 we no longer publish Case Reports; it is replaced by Clinical Vignette.

Submission checklist

You can use this list to carry out a final check of your submission before you send it to the journal for review. Please check the relevant section in this Guide for Authors for more details.

Ensure that the following items are present:
One author has been designated as the corresponding author with contact details:
• E-mail address
• Full postal address

All necessary files have been uploaded:
Manuscript:
• Include keywords
• All figures (include relevant captions)
• All tables (including titles, description, footnotes)
• Ensure all figure and table citations in the text match the files provided
• Indicate clearly if color should be used for any figures in print Graphical Abstracts / Highlights files (where applicable) Supplemental files (where applicable)

Further considerations
• Manuscript has been 'spell checked' and 'grammar checked'
• All references mentioned in the Reference List are cited in the text, and vice versa
• Permission has been obtained for use of copyrighted material from other sources (including the Internet)
• A competing interests statement is provided, even if the authors have no competing interests to declare
• Journal policies detailed in this guide have been reviewed
• Referee suggestions and contact details provided, based on journal requirements

BEFORE YOU BEGIN
Human and animal rights

If the work involves the use of human subjects, the author should ensure that the work described has been carried out in accordance with The Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments involving humans; Uniform Requirements for manuscripts submitted to Biomedical journals. Authors should include a statement in the manuscript that informed consent was obtained for experimentation with human subjects. The privacy rights of human subjects must always be observed.
All animal experiments should comply with the ARRIVE guidelines and should be carried out in accordance with the U.K. Animals (Scientific Procedures) Act, 1986 and associated guidelines, EU Directive 2010/63/EU for animal experiments, or the National Institutes of Health guide for the care and use of Laboratory animals (NIH Publications No. 8023, revised 1978) and the authors should clearly indicate in the manuscript that such guidelines have been followed.

Declaration of conflict of interest

All authors must disclose any financial and personal relationships with other people or organizations that could inappropriately influence (bias) their work. Examples of potential conflicts of interest include employment, consultancies, stock ownership, honoraria, paid expert testimony, patent applications/ registrations, and grants or other funding. Authors must disclose any interests in two places: 1. A summary declaration of interest statement in the title page file (if double-blind) or the manuscript file (if single-blind). If there are no interests to declare then please state this: 'Declarations of interest: none'. This summary statement will be ultimately published if the article is accepted. 2. Detailed disclosures as part of a separate Declaration of Interest form, which forms part of the journal's official records. It is important for potential interests to be declared in both places and that the information matches.

Submission declaration and verification

Submission of an article implies that the work described has not been published previously that it is not under consideration for publication elsewhere, that its publication is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out, and that, if accepted, it will not be published elsewhere in the same form, in English or in any  other language, including electronically without the written consent of the copyright-holder.

Contributors

Each author is required to declare his or her individual contribution to the article: all authors must have materially participated in the research and/or article preparation, so roles for all authors should be described. The statement that all authors have approved the final article should be true and included in the disclosure.

Changes to authorship

Authors are expected to consider carefully the list and order of authors before submitting their manuscript and provide the definitive list of authors at the time of the original submission. Any addition, deletion or rearrangement of author names in the authorship list should be made only before the manuscript has been accepted and only if approved by the journal Editor. To request such a change, the Editor must receive the following from the corresponding author: (a) the reason for the change in author list and (b) written confirmation (e-mail, letter) from all authors that they agree with the addition, removal or rearrangement. In the case of addition or removal of authors, this includes confirmation from the author being added or removed. Only in exceptional circumstances will the Editor consider the addition, deletion or rearrangement of authors after the manuscript has been accepted. While the Editor considers the request, publication of the manuscript will be suspended. If the manuscript has already been published in an online issue, any requests approved by the Editor will result in a corrigendum.

Clinical trial results

In line with the position of the International Committee of Medical Journal Editors, the journal will not consider results posted in the same clinical trials registry in which primary registration resides to be prior publication if the results posted are presented in the form of a brief structured (less than 500 words) abstract or table. However, divulging results in other circumstances (e.g., investors' meetings) is discouraged and may jeopardise consideration of the manuscript. Authors should fully disclose all posting in registries of results of the same or closely related work.

Reporting clinical trials
Randomized controlled trials should be presented according to the CONSORT guidelines. At manuscript submission, authors must provide the CONSORT checklist accompanied by a flow diagram that illustrates the progress of patients through the trial, including recruitment, enrollment, randomization, withdrawal and completion, and a detailed description of the randomization procedure. The CONSORT checklist and template flow diagram are available online.

Registration of clinical trials
Registration in a public trials registry is a condition for publication of clinical trials in this journal in accordance with International Committee of Medical Journal Editors recommendations. Trials must register at or before the onset of patient enrolment. The clinical trial registration number should be included at the end of the abstract of the article. A clinical trial is defined as any  research study that prospectively assigns human participants or groups of humans to one or more health-related interventions to evaluate the effects of health outcomes. Health-related interventions include any intervention used to modify a biomedical or health-related outcome (for example drugs, surgical procedures, devices, behavioural treatments, dietary interventions, and process-of-care changes). Health outcomes include any biomedical or health-related measures obtained in patients or participants, including pharmacokinetic measures and adverse events. Purely observational studies (those in which the assignment of the medical intervention is not at the discretion of the investigator) will not require registration.
Author rights: As an author you (or your employer) have rights to reuse your work.

Role of the funding source

You are requested to identify who provided financial support for the conduct of the research and/or preparation of the article and to briefly describe the role of the sponsor(s), if any, in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the article for publication. If the funding source(s) had no such involvement then this should be stated.

Submission

Submission is possible only via online submission system (mc.manuscriptcentral.com/ahp), which guides you stepwise through the process of entering your article details and uploading your files.
The system converts your article files to a single PDF file used in the peer-review process. Editable files (e.g., Word, LaTeX) are required to typeset your article for final publication. All correspondence, including notification of the Editor's decision and requests for revision, is sent by e-mail.
Reviewers
Please submit the names and institutional e-mail addresses of several potential reviewers. Note that the editor retains the sole right to decide whether or not the suggested reviewers are used.

Permissions

Materials taken from other sources must be accompanied by a written statement from both author and publisher giving permission to AHP for reproduction. Obtain permission in writing from at least one author of papers still in press, unpublished data, and personal communications.

Disclaimer

Every effort is made by the Editor-in-Chief and the Editorial Board of AHP to see that no inaccurate or misleading data, opinion or statement appear in Acta Haematologica Polonica. However, they wish to make it clear that the data and opinions appearing in the articles and advertisements herein are the responsibility of the contributor, sponsor or advertiser concerned. Accordingly, the Editor-in-Chief and the Editorial Board accept no liability whatsoever for the consequences of any such inaccurate of misleading data, opinion or statement. Every effort is made to ensure that drug doses and other quantities are presented accurately. Nevertheless, readers are advised that methods and techniques involving drug usage and other treatments described in journal should only be followed in conjunction with the drug or treatment manufacturer's own published literature in the readers own country.

PREPARATION
NEW SUBMISSIONS

Submission to this journal proceeds totally online and you will be guided stepwise through the creation and uploading of your files. The system automatically converts your files to a single PDF file, which is used in the peer-review process. As part of the Your Paper Your Way service, you may choose to submit your manuscript as a single file to be used in the refereeing process. This should be a Word document, in any format or lay- out that can be used by referees to evaluate your manuscript. It should contain high enough quality figures for refereeing. If you prefer to do so, you may still provide all or some of the source files at the initial submission. Please note that individual figure files larger than 10 MB must be uploaded separately. If your article includes any Videos and/or other Supplementary material, this should be included in your initial submission for peer review purposes. Divide the article into clearly defined sections.

ARTICLE CATEGORIES

The following categories of articles are accepted for publication in Acta Haematologica Polonica:
ORIGINAL ARTICLES: This category contains original studies with appropriate methodology and statistical analysis, ranging from observational to clinical trials. Articles should be limited to 3000 words, up to eight figures or tables, and up to 30 references.
All original articles should contain the following sections: Introduction, Methods, Results, and Discussion. The Methods section of the paper should clearly state that the study was approved by an appropriate institutional review board or ethics committee and that patients provided written informed consent to participate in the study. The abstract should contain maximum 250 words, followed by a list of three to six keywords. All abbreviations used in the abstract should be spelled out at first mention in the text. The number of abbreviations should be restricted to a minimum.
 
REVIEWS: This category contains comprehensive reviews that summarize and critically evaluate current research in the field and identify future implications. Reviews should not exceed 5000 words, 10 figures or tables, and 100 references. They should contain an unstructured abstract containing maximum 250 words and up to 6 key words.
 
EXPERT GUIDELINES: This category contains expert guidelines or recommendations to various aspects related to hematology. Guidelines should not exceed 5000 words, 10 figures or tables, and 100 references. They should contain an unstructured abstract containing maximum 250 words and up to 6 key words.
 
SHORT COMMUNICATIONS: This category ranges from case series to preliminary results or subgroup analyses from clinical trials or observational studies. Short communications do not contain an abstract or keywords and should have no more than 1500 words, 1 figure or table, and up to 15 references. They should contain the following sections: Introduction, Methods, Results and Discussion (combined). The Methods section of the paper should clearly state that the study was approved by an appropriate institutional review board or ethics committee and that patients provided written informed consent to participate in the study.
 
CLINICAL VIGNETTES: This category is intended for brief reports of highly informative cases illustrated with interesting or rare images with detailed legends. Various imaging techniques may be described in this category. Only 1 figure/table is allowed for publication in print, any additional figures may be published as online-only supplementary data. The figure should contain no more than six panels in total (labelled A, B, C, and so on) and arrows illustrating any abnormal findings should be used where possible to improve clarity. The articles should be limited to 750 words and up to 10 references. The submission of videos is encouraged; these will be hosted as supplementary data. The videos may be submitted as mp4 or avi files, their size should not exceed 10 MB.
 
LETTERS TO THE EDITOR: Papers submitted in this category must not exceed 500 words and should focus on a specific article published in Acta Haematologica Polonica within the preceding 6 months. No original data may be included. Letters should have no more than five references. The authors of the article cited will be invited to reply within 14 days. If they decline the invitation or do not respond to it, this information will be provided at the end of the manuscript.
 
EDITORIALS: Papers in this section will comment on the published articles and be solicited by the Editor-in-Chief. The manuscript should not exceed 500 words and 10 references, including a reference to the original article. An abstract and keywords are not required.
 
Case reports are currently not accepted for peer review and publication.

Table. Types of manuscript

Type of manuscript Abstract Word count limit References limit
Original Study ≤250 words 3000  50
Review   ≤250 words 5000 100
Expert Guidelines ≤250 words 5000 100
Short Communication no 1500 15
Clinical Vignette no 750 10
Letter to the Editor no 500 5
Editorial  no 500 10


MANUSCRIPT PREPARATION
Subdivision in sections
Divide your article into clearly defined sections. Any subsection may be given a brief heading. Each heading should appear on its own separate line.
Introduction
State the objectives of the work and provide an adequate background, avoiding a detailed literature survey or a summary of the results.
Material and methods
Provide sufficient details to allow the work to be reproduced by an independent researcher. Methods that are already published should be summarized, and indicated by a reference. If quoting directly from a previously published method, use quotation marks and also cite the source. Any modifications to existing methods should also be described.
Results
Results should be clear and concise.
Discussion
This should explore the significance of the results of the work, not repeat them. A combined Results and Discussion section is often appropriate. Avoid extensive citations and discussion of published literature.
Conclusions
The main conclusions of the study may be presented in a short Conclusions section, which may stand alone or form a subsection of a Discussion or Results and Discussion section.

Essential title page information

Title. Concise and informative. Titles are often used in information-retrieval systems. Avoid abbreviations and formulae where possible.
Author names and affiliations. Please clearly indicate the given name(s) and family name(s) of each author and check that all names are accurately spelled. You can add your name between parentheses in your own script behind the English transliteration. Present the authors' affiliation addresses (where the actual work was done) below the names. Indicate all affiliations with a lower- case superscript letter immediately after the author's name and in front of the appropriate address. Provide the full postal address of each affiliation, including the country name and, if available, the e-mail address of each author.
Corresponding author. Clearly indicate who will handle correspondence at all stages of refereeing and publication, also post-publication. This responsibility includes answering any future queries about Methodology and Materials. Ensure that the e-mail address is given and that contact details are kept up to date by the corresponding author.
Present/permanent address. If an author has moved since the work described in the article was done, or was visiting at the time, a 'Present address' (or 'Permanent address') may be indicated as a footnote to that author's name. The address at which the author actually did the work must be retained as the main, affiliation address. Superscript Arabic numerals are used for such footnotes.

Acknowledgements

Collate acknowledgements in a separate section and include it on the title page only. List here those individuals who provided help during the research (e.g., providing language help, writing assistance or proof reading the article, etc.).

Abstract

A concise and factual abstract is required. The abstract should state briefly the purpose of the research, the principal results and major conclusions. An abstract is often presented separately from the article, so it must be able to stand alone. For this reason, References should be avoided, but if essential, then cite the author(s) and year(s). Also, non-standard or uncommon abbreviations should be avoided, but if essential they must be defined at their first mention in the abstract itself.
Abstract should be included in the Blinded Manuscript. Abstracts for Original Articles should not exceed 200 words and should briefly describe, the purpose of the study, how the investigation was performed, the most important results and the principal conclusion that authors draw from the results.
Keywords
Immediately after the abstract, provide a maximum of 6 keywords, using American spelling and avoiding general and plural terms and multiple concepts (avoid, for example, 'and', 'of'). Be sparing with abbreviations: only abbreviations firmly established in the field may be eligible. These keywords will be used for indexing purposes.
Abbreviations
Define abbreviations that are not standard in this field in a footnote to be placed on the first page of the article. Such abbreviations that are unavoidable in the abstract must be defined at their first mention there, as well as in the footnote. Ensure consistency of abbreviations throughout the article.
Math formulae
Please submit math equations as editable text and not as images. Present simple formulae in line with normal text where possible and use the solidus (/) instead of a horizontal line for small fractional terms, e.g., X/Y. In principle, variables are to be presented in italics. Powers of e are often more conveniently denoted by exp. Number consecutively any equations that have to be displayed separately from the text (if referred to explicitly in the text).
Footnotes
Footnotes should be used sparingly. Number them consecutively throughout the article. Many word processors build footnotes into the text, and this feature may be used. Should this not be the case, indicate the position of footnotes in the text and present the footnotes themselves separately at the end of the article.

Figures

Electronic artwork General points
• Make sure you use uniform lettering and sizing of your original artwork.
• Preferred fonts: Arial (or Helvetica), Times New Roman (or Times), Symbol, Courier.
• Number the illustrations according to their sequence in the text.
• Use a logical naming convention for your artwork files.
• Indicate per figure if it is a single, 1.5 or 2-column fitting image.
• For Word submissions only, you may still provide figures and their captions, and tables within a single file at the revision stage.
• Please note that individual figure files larger than 10 MB must be provided in separate source files.
You are urged to visit this site; some excerpts from the detailed information are given here.

Formats
Regardless of the application used, when your electronic artwork is finalized, please 'save as' or convert the images to one of the following formats (note the resolution requirements for line drawings, halftones, and line/halftone combinations given below):
EPS (or PDF): Vector drawings. Embed the font or save the text as 'graphics'.
TIFF (or JPG): Color or grayscale photographs (halftones): always use a minimum of 300 dpi. TIFF (or JPG): Bitmapped line drawings: use a minimum of 1000 dpi.
TIFF (or JPG): Combinations bitmapped line/half-tone (color or grayscale): a minimum of 500 dpi  is required.

Please do not:
• Supply files that are optimized for screen use (e.g., GIF, BMP, PICT, WPG); the resolution is too low.
• Supply files that are too low in resolution.
• Submit graphics that are disproportionately large for the content.

Figure captions
Ensure that each illustration has a caption. A caption should comprise a brief title (not on the figure itself) and a description of the illustration. Keep text in the illustrations themselves to a minimum but explain all symbols and abbreviations used.

Tables

Please submit tables as editable text and not as images. Tables can be placed either next to the relevant text in the article, or on separate page(s) at the end. Number tables consecutively in accordance with their appearance in the text and place any table notes below the table body. Be sparing in the use of tables and ensure that the data presented in them do not duplicate results described elsewhere in the article. Please avoid using vertical rules and shading in table cells.

Declarations

All manuscripts must contain the following sections at the end of the manuscript, prior to the Reference list:
• Authors' contributions
• Conflict of interests
• Financial support

References

Citation in text
Please ensure that every reference cited in the text is also present in the reference list (and vice versa). Any references cited in the abstract must be given in full. Unpublished results and personal communications are not recommended in the reference list, but may be mentioned in the text. If these references are included in the reference list they should follow the standard reference style of the journal and should include a substitution of the publication date with either 'Unpublished results' or 'Personal communication'. Citation of a reference as 'in press' implies that the item has been accepted for publication.

Web references
As a minimum, the full URL should be given and the date when the reference was last accessed. Any further information, if known (DOI, author names, dates, reference to a source publication, etc.), should also be given. Web references can be listed separately (e.g., after the reference list) under a different heading if desired, or can be included in the reference list.

Data references
This journal encourages you to cite underlying or relevant datasets in your manuscript by citing them in your text and including a data reference in your Reference List. Data references should include the following elements: author name(s), dataset title, data repository, version (where available), year, and global persistent identifier. Add [dataset] immediately before the reference so we can properly identify it as a data reference. The [dataset] identifier will not appear in your published article.

Reference management software
Most journals have their reference template available in many of the most popular reference management software products. These include all products that support Citation Style Language styles, such as EndNote or Mendeley and Zotero. Using the word processor plug-ins from these products, authors only need to select the appropriate journal template when preparing their article, after which citations and bibliographies will be automatically formatted in the journal's style. If no template is yet available for this journal, please follow the format of the sample references and citations as shown in this Guide.
Other software that can be used for reference management: RefWorks (http://www.refworks.com), BibTeX (http://www.bibtex.org), ProCite (http://www.procite.com). Users of Mendeley Desktop can easily install the reference style for this journal by clicking the following link: http://open.mendeley.com/use-citation-style/acta-haematologica-polonica
When preparing your manuscript, you will then be able to select this style using the Mendeley plug-ins for Microsoft Word or LibreOffice.

Reference formatting
Reference style
Indicate references by number(s) in square brackets in line with the text. Number the references (numbers in square brackets) in the list in the order in which they appear in the text.

List of references
List up to 6 authors. For references with more than 6 authors the first 3 should be listed followed by 'et al’. Note shortened form for last page number. e.g., 51–9. For further details you are referred to 'Uniform Requirements for Manuscripts submitted to Biomedical Journals' (J Am Med Assoc 1997;277:927–34) (see also Samples of Formatted References).

Journal abbreviations source
Journal names should be abbreviated according to the List of Title Word Abbreviations.

Examples:
Reference to a journal publication:
[1] Van der Geer J, Hanraads JAJ, Lupton RA, et al. The art of writing a scientific article. J Sci Commun 2020;163:51–9.
Reference to a book:
[2] Strunk Jr W, White EB. The elements of style. 4th ed. New York: Longman; 2020. Reference to a chapter in an edited book:
[3] Mettam GR, Adams LB. How to prepare an electronic version of your article. In: Jones BS, Smith RZ, editors. Introduction to the electronic age, New York: E-Publishing Inc; 2020, p. 281–304.
Reference to a website:
[4] Cancer Research UK. Cancer statistics reports for the UK, http://www.cancerresearchuk.org/aboutcancer/statistics/cancerstatsreport/; 2020 [accessed 10 January 2020].
Reference to a dataset:
[5] Oguro M, Imahiro S, Saito S, Nakashizuka T. Mortality data for Japanese oak wilt disease and surrounding forest compositions, Mendeley Data, v1; 2020. https://doi.org/10.17632/ xwj98nb39r.1.

Supplementary material

Supplementary material such as applications, images and sound clips, can be published with your article to enhance it. Submitted supplementary items are published exactly as they are received (Excel or PowerPoint files will appear as such online). Please submit your material together with the article and supply a concise, descriptive caption for each supplementary file. If you wish to make changes to supplementary material during any stage of the process, please make sure to provide an updated file. Do not annotate any corrections on a previous version. Please switch off the 'Track Changes' option in Microsoft Office files as these will appear in the published version.

REVIEW PROCESS

Manuscripts are evaluated on the basis that they present new insights to the investigated topic, are likely to contribute to a research progress or change in clinical practice or in thinking about a disease. It is understood that all authors listed on a manuscript have agreed to its submission. The signature of the corresponding author on the letter of submission signifies that these conditions have been fulfilled.
Received manuscripts are first examined by the AHP editors. Manuscripts with insufficient priority for publication are rejected promptly. Incomplete packages or manuscripts not prepared in the advised style will be sent back to authors without scientific review. The authors are notified with the reference number upon manuscript registration at the Editorial Office. The registered manuscripts are sent to independent experts for scientific evaluation. We encourage authors to suggest the names of possible reviewers, but we reserve the right of final selection. The evaluation process usually takes 1-3 weeks. Submitted papers are accepted for publication after a positive opinion of the independent reviewers.

Peer review
This journal operates a double blind review process. All contributions will be initially assessed by the editor for suitability for the journal. Papers deemed suitable are then typically sent to a minimum of two independent expert reviewers to assess the scientific quality of the paper. The Editor is responsible for the final decision regarding acceptance or rejection of articles. The Editor's decision is final.

REVISED SUBMISSIONS

Use of word processing software
Regardless of the file format of the original submission, at revision you must provide us with an editable file of the entire article. Keep the layout of the text as simple as possible. Most formatting codes will be removed and replaced on processing the article. The electronic text should be prepared in a way very similar to that of conventional manuscripts.
To avoid unnecessary errors you are strongly advised to use the 'spell-check' and 'grammar-check' functions of your word processor.

RESEARCH DATA

The journal encourages and enables you to share data that supports your research publication where appropriate, and enables you to interlink the data with your published articles. Research data refers to the results of observations or experimentation that validate research findings. To facilitate reproducibility and data reuse, this journal also encourages you to share your software, code, models, algorithms, protocols, methods and other useful materials related to the project.
Below are a number of ways in which you can associate data with your article or make a statement about the availability of your data when submitting your manuscript. If you are sharing data in one of these ways, you are encouraged to cite the data in your manuscript and reference list. Please refer to the "References" section for more information about data citation.
In addition, you can link to relevant data or entities through identifiers within the text of your manuscript, using the following format: Database: xxxx (e.g., TAIR: AT1G01020; CCDC: 734053; PDB: 1XFN).

Mendeley Data
This journal supports Mendeley Data, enabling you to deposit any research data (including raw and processed data, video, code, software, algorithms, protocols, and methods) associated with your manuscript in a free-to-use, open access repository. During the submission process, after uploading your manuscript, you will have the opportunity to upload your relevant datasets directly to Mendeley Data. The datasets will be listed and directly accessible to readers next to your published article online.

Data statement
To foster transparency, we encourage you to state the availability of your data in your submission. This may be a requirement of your funding body or institution. If your data is unavailable to access or unsuitable to post, you will have the opportunity to indicate why during the submission process, for example by stating that the research data is confidential. The statement will appear with your published article on Sciendo open access platform.

poprzedni artykuł