compS :: Spacer -> Spacer -> Spacer idS :: Spacer flipS :: Spacer -> Spacer
idS is the identity spacer, i.e. a spacer that doesn't affect the
spacing.
compS is spacer composition.
flipS transforms a spacer into a mirror symmetric version with respect
to the line x=y.
flipS (flipS spacer) = spacer idS `compS` spacer = spacer spacer `compS` idS = spacer
