 The term \(\textsf{S}(\textsf{KS})\textsf{K}\) is in nf, however, to be in nf is not a requirement for definability. It is more convenient to work with defining terms that are in nf, because an application of a combinator that is not in nf could be started with reducing the combinator to its normal form. (Also, there are always infinitely many combinators that reduce to a combinator.) However, note that the preference for choosing combinators in nf is not meant to imply that a combinator cannot be defined by two or more terms in nf; below we give two definitions (involving only \(\textsf{S}\) and \(\textsf{K}\)) for \(\textsf{I}\).