The containment operator a in b aggregates the logical or of a == b.

Typical examples are a in (1 : 3) which has true as result if and only if a equals 1, 2 or 3, or a in n.children() which tests whether a node a is contained in the sequence of values returned by the generator method n.children(). The containment operator terminates the evaluation of the generator expression once it knows the final result, i. e., if a == b has been true for some pair a, b.

The containment operator, like instanceof, does not follow the general pattern that only symbols and no keywords are used for operators.