macro (x += y) `(~x = ~x + ~y)
macro (x -= y) `(~x = ~x - ~y)

- (n: Number) := -1 * n

(n: Integer) even? := 2 divides?: n
(n: Integer) odd? := n even? not

(x: Integer) divides?: (y: Integer) :=
  (y % x) == 0

(x: Integer) divisible-by?: (y: Integer) :=
  y divides?: x

(n: Number) abs :=
  if: (n < 0)
    then: { - n }
    else: { n }