recognizer for booleans
Major Section:  ACL2-BUILT-INS

(Booleanp x) is t if x is t or nil, and is nil otherwise.

See generalized-booleans for a discussion of a potential soundness problem for ACL2 related to the question: Which Common Lisp functions are known to return Boolean values?

To see the ACL2 definition of this function, see pf.