(declare expr)

(def fact
  (a/combine decimal
             (<* (*> (p-char \() expr) (p-char \)))))

(def term
  (chainl1 fact (a/combine (<$ * (p-char \*))
                           (<$ * (p-char \/)))))

(def expr
  (chainl1 term (a/combine (<$ + (p-char \+))
                           (<$ - (p-char \-)))))