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