(defun ebi-lang-p (s)
  (loop with stack = ()
        for c across s
        do (cond ((char= c #\A)
                  (push c stack))
                 ((char= c #\B)
                  (if stack
                      (pop stack)
                      ;; 対応する A のない B に出会した
                      (return nil)))
                 (t
                  ;; A と B 以外を含む場合は偽とする
                  (return nil)))
        finally (return (null stack))))
 
(loop for s in '("" "A" "B" "AB" "ABB" "AAB" "AABB" "ABAABBAAABBB" "ABC")
      for v = (ebi-lang-p s)
      do (format t "~A → ~:[🐟~;🦐~]~%" s v))
 

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