Fixed Point Combinator

A fixed point combinator is a higher-order function that computes a fixed point of other functions. A fixed point of a function f is a value x such that f(x) = x. For example, 0 and 1 are fixed points of the function f(x) = x2, because 02 = 0 and 12 = 1. Whereas a fixed-point of a first-order function is a first-order value, a fixed point of a higher-order function f is another function p such that f(p) = p