cons (a:as) x
          | a == "+" = cons as (Elem (Sum y y') x'')
        | a == "*" = cons as (Elem (Mul y y') x'')
        | otherwise = cons as (Elem (Atom (read a::Integer)) x)
          where x' = pop x
                x'' = pop x'
                y = giveTop x
                y' = giveTop x'