I should be able to check whether a received value is contained in a Collection

(User Story, Closed -> Fixed, Priority: Critical, Test Status: No automated tests yet , Reported By Bad Link: Thing 3y284oe not found, )
Functions
Summary: This is basically the inverse of _contains(). It isn't strictly necessary, but makes some expressions cleaner.
This is a pretty simple function, and it makes sense to have it, so I'm implementing it as _isContainedIn().