Second induction step of the completeness lemma for parse-alternation.
Theorem:
(defthm parse-alternation-when-tree-match-induction-step-2 (implies (and (not (mv-nth 0 (parse-concatenation input))) (< (len (mv-nth 2 (parse-concatenation input))) (len input)) (pred-concatenation input) (pred-alt-rest (mv-nth 2 (parse-concatenation input)))) (pred-alternation input)))