Input decomposition theorem for parse-1*-dot-1*digit.
Theorem:
(defthm input-decomposition-of-parse-1*-dot-1*digit (b* (((mv error? trees rest-input) (parse-1*-dot-1*digit input))) (implies (not error?) (equal (append (tree-list->string trees) rest-input) (nat-list-fix input)))))