 "N") would do if "x" has no free occurrence in "N".S passes the argument on to both subterms of the application, and then applies the result of the first to the result of the second.The combinators B and C are similar to S, but pass the argument on to only one subterm of an application (B to the "argument" subterm and C to the "function" subterm), thus saving a subsequent K if there is no occurrence of "x" in one subterm.