issues about quantification in ACL2
Major Section:  DEFUN-SK

ACL2 supports first-order quantifiers exists and forall by way of the defun-sk event. However, proof support for quantification is quite limited. Therefore, we recommend using recursion in place of defun-sk when possible (following common ACL2 practice).

For example, the notion ``every member of x has property p'' can be defined either with recursion or explicit quantification, but proofs may be simpler when recursion is used. We illustrate this point with two proofs of the same informal claim, one of which uses recursion which the other uses explicit quantification. Notice that with recursion, the proof goes through fully automatically; but this is far from true with explicit quantification (especially notable is the ugly hint).

The informal claim for our examples is: If every member a of each of two lists satisfies the predicate (p a), then this holds of their append; and, conversely.

See quantifiers-using-recursion for a solution to this example using recursion.

See quantifiers-using-defun-sk for a solution to this example using defun-sk. Also See quantifiers-using-defun-sk-extended for an elaboration on that solution.