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 reductionSyStemS, a function which returnSSome component of itS argument. E.g. head, tail, (x,y) . x. In a graph reductionSyStem the function can juSt return a pointer to part of itS argument and doeS not need to build any new graph. (1997-01-29)
