Implemented by: Object, Collection, Nil, Function
matchItem(item) may be passed to different objects that behave as constraints. More Objects may be conceived to implement matchItem to extend the interface.
See also: Dictionary: -matchAt.
Test if item is identical to object.
Test if item is included in collection.
returns true (Nil serves as a "joker", a stand-in for anything).
Test item by passing it to a function which should return a Boolean.