 Addition may be defined by the recursive equation \(+mn=Zmn (\textsf{V}(\textsf{KI})(+(Pm)n))\), where \(m\) and \(n\) are numerals, and \(P\) and \(Z\) are the already defined functions. (The abbreviations are used to enhance the readability of the terms—they can be replaced everywhere by the defining combinators.) To put into words, if \(m\) is \(0\) then the sum is \(n\), otherwise \(m+n\) is the successor of \((m-1)+n\). Of course, this definition closely simulates the definition of addition from recursion theory, where addition is often defined by the two equations \(+(0,n)=n\) and \(+(s(m),n)=s(+(m,n))\) (with \(s\) denoting the successor function). The fact that CL can express addition in this form shows—once again—the versatility of CL.