data Matter = Solid | Liquid | Gas

interface MyEq ty where
    (==) : ty -> ty -> Bool
    (/=) : ty -> ty -> Bool

MyEq Matter where
      (==) Solid Solid = True
      (==) Liquid Liquid = True
      (==) Gas Gas = True
      (==) _ _ = False

      (/=) x y = not (x == y)

occurrences : MyEq ty => (item :ty) -> (values : List ty) -> Nat
occurrences item [] = 0
occurrences item (value :: values) = case value == item of
                                          False => occurrences item values
                                          True => 1 + occurrences item values