substitute :: Expr -> Char -> Double -> Expr
  substitute (Mult  a b) x value = Mult  (substitute a x value) (substitute b x value)
substitute :: Expr -> Char -> Double -> Expr
substitute (Mult  a b) x value = Mult  (substitute a x value) (substitute b x value)
substitute (Mult  a b) x value = Mult  (substitute a x value) (substitute b x value)