 Not all terms have an nf, however, many do. Examples, first of all, include \(\textsf{S}\) and \(\textsf{K}\). (The variables, if included, of which there are \(\aleph_0\) many, are all in nf.) None of these terms contains a redex, hence each reduces only to itself. By the Church–Rosser theorem, it is excluded that some term \(M\) could reduce to both \(x\) and \(\textsf{S}\) (making \(\textsf{S}\) equal to \(x\)).