data Color = Red | Green | Blue

succ Red = Green
succ Green = Blue

pred Blue = Green
pred Green = Red

eqColor Red Red = True
eqColor Red Green = False
eqColor Red Blue = False
eqColor Green Red = False
eqColor Green Green = True
eqColor Green Blue = False
eqColor Blue Red = False
eqColor Blue Green = False
eqColor Blue Blue = True