Examples
(expr '(a + b) ) ==> (+ A B) (expr '(x := a + b * c) ) ==> (:= X (+ A (* B C))) (expr '(x := a * b + c) ) ==> (:= X (+ (* A B) C)) (expr '(x := (a + b) * (c + d * e) + f) ) ==> (:= X (+ (* (+ A B) (+ C (* D E))) F))