First induction step of the completeness lemma for parse-alt-rest.
Theorem: parse-alt-rest-when-tree-list-match-induction-step-1
(defthm parse-alt-rest-when-tree-list-match-induction-step-1 (implies (and (mv-nth 0 (parse-alt-rest-comp input)) (pred-alt-rest-comp input)) (pred-alt-rest input)))