[en] For a class C of Boolean functions, a Boolean function f is a C-subfunction of a Boolean function g, if f=g(h1,...,hn), where all the inner functions hi are members of C. Two functions are C-equivalent, if they are C-subfunctions of each other. The C-subfunction relation is a preorder on the set of all functions if and only if C is a clone. An infinite descending chain of U∞-subfunctions is constructed from certain threshold functions (U∞ denotes the clone of clique functions).
