Keywords
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Verbs
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`RFC2119 <https://datatracker.ietf.org/doc/html/rfc2119>`_ defines the meaning of common verbs:

- **MUST**, **REQUIRED**, **SHALL**: absolute requirement of the specification.
- **MUST NOT**, **SHALL NOT**: absolute prohibition of the specification.
- **SHOULD**, **RECOMMENDED**: there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed.
- **SHOULD NOT**, **NOT RECOMMENDED**: there may exist valid reasons in particular circumstances when the particular behavior is acceptable or useful, but the full implications must be understood and carefully weighted.
- **MAY**, **OPTIONAL**: the item is truly optional. Implementations must be ready to accomodate for the missing item.