Predicates can be combined by the logical operators ` AND`, ` OR`,
and ` NOT`.

` (NOT x)` returns ` T` if ` x` is ` NIL`; otherwise, it
returns ` NIL`. ` NOT` is therefore the same as ` NULL`.

` (AND x_{1} ... x_{n})` evaluates each of

(DEFUN SAFE-SQRT (X) (AND (NUMBERP X) (NOT (MINUSP X)) (SQRT X)) )

` (OR x_{1} ... x_{n})` evaluates each of

(OR (> CASH 100) (PRINT ``Get money.''))