validCoverageCase ctxt (CantConvert topx topy _)
    = let topx' = normalise ctxt [] topx
          topy' = normalise ctxt [] topy in
          not (sameFam topx' topy')
  where sameFam topx topy
            = case (unApply topx, unApply topy) of
                   ((P _ x _, _), (P _ y _, _)) -> x == y
                   _ -> False
validCoverageCase ctxt (CantConvert topx topy _)
    = let topx' = normalise ctxt [] topx
          topy' = normalise ctxt [] topy in
          not (sameFam topx' topy')
  where sameFam topx topy
            = case (unApply topx, unApply topy) of
                   ((P _ x _, _), (P _ y _, _)) -> x == y
                   _ -> False
validCoverageCase ctxt (CantConvert topx topy _)
    = let topx' = normalise ctxt [] topx
          topy' = normalise ctxt [] topy in
          not (sameFam topx' topy')
  where sameFam topx topy
            = case (unApply topx, unApply topy) of
                   ((P _ x _, _), (P _ y _, _)) -> x == y
                   _ -> False