interactInertSub :: [Xi] -> Xi -> [Xi] -> Xi -> [Pair Type]
interactInertSub [x1,y1] z1 [x2,y2] z2
  | sameZ && tcEqType x1 x2         = [ y1 === y2 ]
  | sameZ && tcEqType y1 y2         = [ x1 === x2 ]
  where sameZ = tcEqType z1 z2
interactInertSub :: [Xi] -> Xi -> [Xi] -> Xi -> [Pair Type]
interactInertSub [x1,y1] z1 [x2,y2] z2
  | sameZ && tcEqType x1 x2         = [ y1 === y2 ]
  | sameZ && tcEqType y1 y2         = [ x1 === x2 ]
  where sameZ = tcEqType z1 z2
interactInertSub [x1,y1] z1 [x2,y2] z2
  | sameZ && tcEqType x1 x2         = [ y1 === y2 ]
  | sameZ && tcEqType y1 y2         = [ x1 === x2 ]
  where sameZ = tcEqType z1 z2