f(f(x))=f(x) and (b) whose result is no more defined than its argument.

E.g. F(x)=bottom or F(x)=x.

In reduction systems, a function which returns some component of its argument.

E.g. head, tail, \ (x,y) . x. In a graph reduction system the function can just return a pointer to part of its argument and does not need to build any new graph.