This document defines the Concat notation: a text-based language used to describe pictures and videos whose subject includes cats, containers, and their interactions.

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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 [RFC 0020], allowing the transfer of cat-related materials in environments with restricted capabilities.

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 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

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.

The grammar is defined using the ABNF notation [RFC 5234].

The standard notation for a cat is the word cat.

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.

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.

Balls of yarn SHOULD be represented with @.

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.

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.

The + operator is used to represent subjects or containers next to each other.

When a subject is above or on top of another, the operator / MUST be used.

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.

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.

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.

The word cat is in English and is provided to allow transfer of Concat notations using only the US-ASCII character encoding [RFC 0020]. 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.

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.

This document has no IANA actions.

[RFC0020]

V.G. Cerf, "ASCII format for network interchange", STD 80, RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.

[RFC2119]

S. Bradner, "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]

D. Crocker, 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]

B. Leiba, "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>.

This appendix provides some examples of the Concat notation.

Mattia Basaglia

Joep Bernards

Joost Maas