interactInertMul :: [Xi] -> Xi -> [Xi] -> Xi -> [Pair Type]
interactInertMul [x1,y1] z1 [x2,y2] z2
  | sameZ && known (/= 0) x1 && tcEqType x1 x2 = [ y1 === y2 ]
  | sameZ && known (/= 0) y1 && tcEqType y1 y2 = [ x1 === x2 ]
  where sameZ   = tcEqType z1 z2
interactInertMul :: [Xi] -> Xi -> [Xi] -> Xi -> [Pair Type]
interactInertMul [x1,y1] z1 [x2,y2] z2
  | sameZ && known (/= 0) x1 && tcEqType x1 x2 = [ y1 === y2 ]
  | sameZ && known (/= 0) y1 && tcEqType y1 y2 = [ x1 === x2 ]
  where sameZ   = tcEqType z1 z2
interactInertMul [x1,y1] z1 [x2,y2] z2
  | sameZ && known (/= 0) x1 && tcEqType x1 x2 = [ y1 === y2 ]
  | sameZ && known (/= 0) y1 && tcEqType y1 y2 = [ x1 === x2 ]
  where sameZ   = tcEqType z1 z2