A state that encapsulates a set of {#requirements} with an {Array} of possibilities
A state that encapsulates a single possibility to fulfill the given {#requirement}
Contains or links to the content of the Entry. It has the following attributes:
type
src
Reference: validator.w3.org/feed/docs/rfc4287.html#element.content
The email of the person or entity.
Reference: validator.w3.org/feed/docs/rfc4287.html#element.email