 If functions that are indistinguishable from the point of view of their input–output behavior are to be identified, that is, an extensional understanding of functions is sought, then \(\text{CL}_\triangleright\) and \(\text{CL}_=\) have to be extended by the following rule, (where the symbol \(\ddagger\) is to be replaced by \(\triangleright\) or \(=\), respectively).