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 x₁ ... x_n) evaluates each of x₁ ... x_n in order. As soon as any x_i returns NIL, AND returns NIL without evaluating any remaining x's. If every x_i returns a non- NIL value, the value of AND is the value of x_n.

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

(OR x₁ ... x_n) evaluates each of x₁ ... x_n in order. As soon as any x_i returns a non- NIL value, OR returns that value without evaluating any remaining x's. If every x_i returns NIL, the value of OR is NIL.

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

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