<theory> 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
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