type mode =
    Normal
  | Multiply
  | Screen
  | Overlay
  | ColorDodge
  | ColorBurn
  | Darken
  | Lighten
  | Difference
  | Exclusion

val blend : mode -> int -> int -> int
    (* [blend blendmode src dst] *)

val f : mode -> int -> Color.rgb -> Color.rgb -> Color.rgb
    (* [f blendmode srcalpha src dst] 8bit depth *)
    (* bug: no dstination alpha support *)
