filterSigNames p (FixSig (FixitySig ns ty)) =
  case filter (p . unLoc) ns of
    []       -> Nothing
    filtered -> Just (FixSig (FixitySig filtered ty))
filterSigNames p (FixSig (FixitySig ns ty)) =
  case filter (p . unLoc) ns of
    []       -> Nothing
    filtered -> Just (FixSig (FixitySig filtered ty))
filterSigNames p (FixSig (FixitySig ns ty)) =
  case filter (p . unLoc) ns of
    []       -> Nothing
    filtered -> Just (FixSig (FixitySig filtered ty))