contains?

(contains? coll k)

Answers whether k is present: a key of the map, a member of the set, or a valid index of the vector or string (a bounds check). nil contains nothing. A Java Map or Set looks k up itself, as get does; any other Java object signals, like the oracle. Anything else (a list, a seq, a keyword, a number, false) signals IllegalArgumentException, like the oracle.

Deviation: a string answers false for a key that is no integer index, where the oracle truncates a number and signals for any other key.

As a value a two-argument lambda.