In domain theory, a function, f, which is (a) idempoteNT, i.e. 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. (1997-01-29)