matchFamSub :: [Type] -> Maybe (CoAxiomRule, [Type], Type)
matchFamSub [s,t]
  | Just 0 <- mbY = Just (axSub0R, [s], s)
  | Just x <- mbX, Just y <- mbY, Just z <- minus x y =
    Just (axSubDef, [s,t], num z)
  where mbX = isNumLitTy s
        mbY = isNumLitTy t
matchFamSub :: [Type] -> Maybe (CoAxiomRule, [Type], Type)
matchFamSub [s,t]
  | Just 0 <- mbY = Just (axSub0R, [s], s)
  | Just x <- mbX, Just y <- mbY, Just z <- minus x y =
    Just (axSubDef, [s,t], num z)
  where mbX = isNumLitTy s
        mbY = isNumLitTy t
matchFamSub [s,t]
  | Just 0 <- mbY = Just (axSub0R, [s], s)
  | Just x <- mbX, Just y <- mbY, Just z <- minus x y =
    Just (axSubDef, [s,t], num z)
  where mbX = isNumLitTy s
        mbY = isNumLitTy t