 The theorem’s assumption may be strengthened to exclude the possibility that some occurrences of \(x\) do not occur in \(M\). Then the consequent may be strengthened by adding the qualification that \(\textsf{X}\) is a relevant combinator, more specifically, \(\textsf{X}\) is a combinator over \(\{\textsf{B},\textsf{W}, \textsf{C},\textsf{I}\}\) (a base that does not contain a combinator with cancellative effect), or equivalently, \(\textsf{X}\) is a combinator over \(\{\textsf{I},\textsf{J}\}\).  (These bases correspond to Church’s preferred \(\lambda\textsf{I}\)-calculus.)