plusFM (FM x) (FM y) = FM (M.union y x)
plusFM_C f (FM x) (FM y) = FM (M.unionWith f x y)
minusFM (FM x) (FM y) = FM (M.difference x y)
plusFM (FM x) (FM y) = FM (M.union y x)
plusFM_C f (FM x) (FM y) = FM (M.unionWith f x y)
minusFM (FM x) (FM y) = FM (M.difference x y)