 The relation \(\triangleright\) is called weak reduction, and it may be  defined alternatively as follows. (‘Weak reduction’ is a technical term used in CL to distinguish this relation on the set of terms from some other relations, one of which is called ‘strong reduction’.) A term that is either of the form \(\textsf{S}MNP\) or of the form  \(\textsf{K}MN\) is a redex, and the leading combinators (\(\textsf{S}\) and \(\textsf{K}\), respectively) are the heads of the redexes. If a term \(Q\) contains a subterm of the form \(\textsf{S}MNP\), then \(Q^\prime\); which is obtained by replacing that subterm by \(MP(NP)\) is a one-step reduct of \(Q\). (Similarly, for the redex \(\textsf{K} MN\) and \(M\).) That is, \(Q\triangleright Q^\prime\) in both cases.  Reduction then may be defined as the reflexive transitive closure of one-step reduction. This notion is completely captured by \(\text{CL}_\triangleright\). The calculus \(\text{CL}_\triangleright\) is complete in the sense that if \(M\triangleright N\) in the sense we have just described, then \(\text{CL}_\triangleright\) proves \(M\triangleright N\).  (It is easy to see that the converse implication is true too.)