data Black = Black

data BW > Black = White

data Color > BW = Red

struct Point =
    x :: Int
    y :: Int

struct CPoint < Point =
    col :: Color

colnr :: Color -> Int
colnr c = 1

pt  = {x = 1; y = 2}

cpt = {x= 1; y = 2; col = Red}

match Black = True
match White = False

cmatch Black = True
cmatch White = False
cmatch Red   = False

addpt p = p.x + p.y

addcpt p = p.x + (colnr p.col)

mkbw True  = Black
mkbw False = White

mkcol True  = Black
mkcol False = Red

-- should fail:
--
-- {x = 1; col = Red}

-- will currently not fail:
incomplete Black = 1
incomplete Red   = 3

f p = if cmatch p.col then p else pt