The solution is not to stick strictly to prefix, postifx, or infix notation. Instead, all three should be valid. All these should be equivalent: + a b + c d

The solution is not to stick strictly to prefix, postifx, or infix notation. Instead, all three should be valid. All these should be equivalent: + a b + c d + + + + a b c d a + b + c + d a b c d + + + More insidiously, these should be equivalent: x = a + b = + x a b x a b = + + x = a b x = a b + x a + b =