data Atree = Number Int | Variable String | Plus Atree Atree | Minus Atree | Times Atree Atree

t :: Atree
t =
  Times
    (Plus (Number 5) (Minus (Variable "x")))
    ( Plus
        (Times (Number 2) (Variable "y"))
        (Minus (Plus (Variable "x") (Number 7)))
    )

type Polynomial = [Monomial]

type Monomial = (Int, [String])

polynomial :: Atree -> Polynomial
polynomial = undefined