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