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+Independent Submission                                       M. Basaglia
+Request for Comments: 9402                                              
+Category: Informational                                      J. Bernards
+ISSN: 2070-1721                                                         
+                                                                 J. Maas
+                                                            1 April 2023
+
+
+                            Concat Notation
+
+Abstract
+
+   This document defines the Concat notation: a text-based language used
+   to describe pictures and videos whose subject includes cats,
+   containers, and their interactions.
+
+Status of This Memo
+
+   This document is not an Internet Standards Track specification; it is
+   published for informational purposes.
+
+   This is a contribution to the RFC Series, independently of any other
+   RFC stream.  The RFC Editor has chosen to publish this document at
+   its discretion and makes no statement about its value for
+   implementation or deployment.  Documents approved for publication by
+   the RFC Editor are not candidates for any level of Internet Standard;
+   see Section 2 of RFC 7841.
+
+   Information about the current status of this document, any errata,
+   and how to provide feedback on it may be obtained at
+   https://www.rfc-editor.org/info/rfc9402.
+
+Copyright Notice
+
+   Copyright (c) 2023 IETF Trust and the persons identified as the
+   document authors.  All rights reserved.
+
+   This document is subject to BCP 78 and the IETF Trust's Legal
+   Provisions Relating to IETF Documents
+   (https://trustee.ietf.org/license-info) in effect on the date of
+   publication of this document.  Please review these documents
+   carefully, as they describe your rights and restrictions with respect
+   to this document.
+
+Table of Contents
+
+   1.  Introduction
+     1.1.  Conventions Used in This Document
+   2.  Definition
+     2.1.  Terminology
+     2.2.  Grammar
+   3.  Elements
+     3.1.  Subjects
+       3.1.1.  Cats
+       3.1.2.  Partial Cats
+       3.1.3.  Other Animals
+       3.1.4.  Balls of Yarn
+     3.2.  Containers
+     3.3.  Positioning
+       3.3.1.  Horizontal Position
+       3.3.2.  Vertical Position
+       3.3.3.  Multiple Repeated Objects
+     3.4.  Changes over Time
+       3.4.1.  Disambiguation
+   4.  Internationalization Considerations
+   5.  Security Considerations
+   6.  IANA Considerations
+   7.  Normative References
+   Appendix A.  Examples
+   Authors' Addresses
+
+1.  Introduction
+
+   Cat pictures and videos are often shared across the Internet.  Many
+   of these files display feline subjects interacting with boxes and
+   other containers.
+
+   Since there is currently no compact notation for describing such
+   media, this document details a standard notation to describe the
+   position and interaction of cats, containers, and related subjects
+   pictured in these images.
+
+   The notation language described in this document is text-based and
+   limits itself to the US-ASCII character encoding [RFC0020], allowing
+   the transfer of cat-related materials in environments with restricted
+   capabilities.
+
+1.1.  Conventions Used in This Document
+
+   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
+   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
+   "OPTIONAL" in this document are to be interpreted as described in
+   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
+   capitals, as shown here.
+
+2.  Definition
+
+2.1.  Terminology
+
+   This document uses specific terms to refer to items being depicted by
+   the notation described herein.
+
+   To avoid ambiguity, such terms are defined as follows:
+
+   Subject:  The term "subject" is used in this document to refer to the
+      object that is the focus in the media to be annotated.  This
+      usually is an animate object, specifically a cat.  An annotation
+      can have multiple subjects interacting in various ways.
+
+   Cat:  A cat is a special kind of subject of feline origin.  This
+      document will assume a house cat is present in the source media;
+      however, other felines are also acceptable.
+
+   Container:  The term "container" is used to refer to inanimate
+      objects inside of which one or more subjects can be located.  Most
+      commonly, this will be a cardboard box; however, a variety of
+      containers can be used.
+
+2.2.  Grammar
+
+   The grammar is defined using the ABNF notation [RFC5234].
+
+   SEQUENCE  =  POSITION / POSITION "=>" SEQUENCE
+   POSITION  =  ADJACENT
+   ADJACENT  =  OVER / ADJACENT "+" OVER
+   OVER      =  MULTIPLE / MULTIPLE "/" POSITION
+   MULTIPLE  =  CONCAT / NUMBER [ "*" ] MULTIPLE / NUMBER "/" MULTIPLE
+   CONCAT    =  SUBJECT [ NUMBER ] / [ PARTIAL ] CONTAINER [ PARTIAL ]
+   CONTAINER =  "[" OPT-POS "]" / "(" OPT-POS ")"
+   CONTAINER =/ "{" OPT-POS "}" / "<" OPT-POS ">"
+   OPT-POS   =  [ POSITION ]
+   SUBJECT   =  CAT / 1*ALPHA / "@"
+   CAT       =  "cat" / PARTIAL
+   PARTIAL   =  "c" / "a" / "t" / "ca" / "at"
+   ALPHA     =   %x41-5A / %x61-7A
+   NUMBER    =  1*DIGIT
+   DIGIT     =  "0" / "1" / "2" / "3" / "4"
+   DIGIT     =/ "5" / "6" / "7" / "8" / "9"
+
+3.  Elements
+
+3.1.  Subjects
+
+3.1.1.  Cats
+
+   The standard notation for a cat is the word cat.
+
+3.1.2.  Partial Cats
+
+   When referencing cats partly inside a container, the annotation MUST
+   contain the full cat mark adequately split inside and outside the
+   container.
+
+   If a cat is only partly visible in the frame of the picture or video,
+   the annotation MAY only reference the visible portion of the cat.
+
+   The partial cat notations are as follows:
+
+   c:  marks the head of the cat.
+
+   a:  marks the body of the cat.
+
+   t:  marks the tail of the cat.
+
+   ca:  marks the head and body of the cat.
+
+   at:  marks the body and tail of the cat.
+
+   The annotation for a partial cat SHOULD use the terms mentioned above
+   that best describe the portion of the cat that is being referenced.
+
+3.1.3.  Other Animals
+
+   Other animals or animate objects SHOULD be represented with a
+   suitable word describing the species of such animal.  The cat-
+   specific words described in this document MUST NOT be used for non-
+   feline subjects.
+
+3.1.4.  Balls of Yarn
+
+   Balls of yarn SHOULD be represented with @.
+
+3.2.  Containers
+
+   When a cat or other subject is inside a container, the container
+   notation MUST be used.  Such notation is denoted by its subject being
+   between brackets.  The type of bracket depends on the shape of the
+   container as follows:
+
+   *  Square brackets represent boxes or other containers with a
+      rectangular opening.
+
+   *  Parentheses represent containers with a round opening or shape.
+
+   *  Curly braces SHALL be used to represent soft containers without a
+      fixed shape.
+
+   Additionally, angle brackets MAY be used to group subjects outside a
+   container.  Such annotations MUST NOT contain partial cats.
+
+3.3.  Positioning
+
+   The Concat notation only gives information about the general layout
+   of subjects and containers, but it does make a distinction between
+   horizontal and vertical positions.
+
+   The order of positional operands SHOULD follow the order in which
+   they appear from left to right in the source media.
+
+3.3.1.  Horizontal Position
+
+   The + operator is used to represent subjects or containers next to
+   each other.
+
+3.3.2.  Vertical Position
+
+   When a subject is above or on top of another, the operator / MUST be
+   used.
+
+3.3.3.  Multiple Repeated Objects
+
+   When multiple objects or configurations are repeated, the shorthand
+   notation MAY be used.
+
+   Horizontal positioning is denoted by a number followed by an optional
+   * and the annotation to be repeated.
+
+   Similarly, for vertical positioning, repeated objects are denoted by
+   a number followed by / and the annotation to be repeated.
+
+   When using such a shorthand, the number of repetitions MUST be a
+   positive integer.
+
+3.4.  Changes over Time
+
+   In the case of videos or other animations, a proper Concat notation
+   SHOULD make use of the state change operator (=>) to mark significant
+   changes in the cat position and major interactions.
+
+3.4.1.  Disambiguation
+
+   Subject tokens MAY be followed by an integer identifier to
+   distinguish specific cats, balls of yarn, or other subjects.  An
+   annotation containing such numeric disambiguations MUST contain such
+   disambiguations for all cats and balls of yarn.
+
+   Since a specific subject can only appear once in a static image,
+   disambiguation identifiers SHOULD be used only on annotations showing
+   state changes.
+
+4.  Internationalization Considerations
+
+   The word cat is in English and is provided to allow transfer of
+   Concat notations using only the US-ASCII character encoding
+   [RFC0020].
+
+   Users of other languages MAY extend the alphabet and use their
+   localized words for cat and other animals.
+
+   Non-standard words for cats SHOULD NOT be used unless all parties
+   involved in the production and consumption of the Concat notation
+   have agreed upon a character encoding and a language prior to the
+   transmission of the annotation.
+
+5.  Security Considerations
+
+   A cat might find themselves in a container smaller than the perceived
+   volume of the cat.  While this might seem to be a dangerous
+   situation, it's actually a natural occurrence when the cat is in its
+   liquid form.
+
+   Cats might chew on the cardboard of the box containing them.  To
+   mitigate this attack, we recommend having multiple boxes to put the
+   cats into.
+
+6.  IANA Considerations
+
+   This document has no IANA actions.
+
+7.  Normative References
+
+   [RFC0020]  Cerf, V., "ASCII format for network interchange", STD 80,
+              RFC 20, DOI 10.17487/RFC0020, October 1969,
+              <https://www.rfc-editor.org/info/rfc20>.
+
+   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
+              Requirement Levels", BCP 14, RFC 2119,
+              DOI 10.17487/RFC2119, March 1997,
+              <https://www.rfc-editor.org/info/rfc2119>.
+
+   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
+              Specifications: ABNF", STD 68, RFC 5234,
+              DOI 10.17487/RFC5234, January 2008,
+              <https://www.rfc-editor.org/info/rfc5234>.
+
+   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
+              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
+              May 2017, <https://www.rfc-editor.org/info/rfc8174>.
+
+Appendix A.  Examples
+
+   This appendix provides some examples of the Concat notation.
+
+   [cat]
+
+                          Figure 1: A Cat in a Box
+
+   [cat] + cat
+
+            Figure 2: A Cat in a Box Next to a Cat Not in a Box
+
+   cat / [cat]
+
+             Figure 3: A Cat over a Box Containing Another Cat
+
+   [c]at
+
+                 Figure 4: A Cat with Its Head inside a Box
+
+   3 * cat
+
+                       Figure 5: 3 Cats Side by Side
+
+   3 / cat
+
+                   Figure 6: 3 Cats on Top of Each Other
+
+   cat + cat / [cat]
+
+      Figure 7: A Cat Standing Next to a Box That Has a Cat on Top and
+                                inside of It
+
+   <cat + cat> / [cat]
+
+     Figure 8: Two Cats Standing on a Box with Another Cat inside of It
+
+   cat1 + [cat2] => cat2 + [cat1]
+
+         Figure 9: A Cat inside a Box and a Cat outside Swap Places
+
+Authors' Addresses
+
+   Mattia Basaglia
+   Email: glax@dragon.best
+   URI:   https://dragon.best/
+
+
+   Joep Bernards
+   Email: joep@duali.xyz
+
+
+   Joost Maas
+   Email: J.f.w.maas@tue.nl