Abstract supertype of objects that contain other values, called elements, where it is possible to efficiently determine if a given value is an element. Category does not satisfy Container, because it is conceptually possible to have a Category whose emptiness cannot be computed.

The in operator may be used to determine if a value belongs to a Category:

if ("hello" in "hello world") { ... }
if (69 in 0..100) { ... }
if (key->value in { for (n in 0..100) n.string->n**2 }) { ... }

Ordinarily, x==y implies that x in cat == y in cat. But this contract is not required since it is possible to form a meaningful Category using a different equivalence relation. For example, an IdentitySet is a meaningful Category.