Base case of the completeness lemma for parse-alt-rest-comp.
Theorem: parse-alt-rest-comp-when-tree-match-base-case
(defthm parse-alt-rest-comp-when-tree-match-base-case (implies (mv-nth 0 (parse-ichar #\/ (mv-nth 2 (parse-*cwsp input)))) (pred-alt-rest-comp input)))