Allowed elements in a set.

gistfile1.hrl

-module(test).
-export([allowed/1]).

% null case
allowed([]) ->
	[];

% single element case
allowed([E]) ->
	[[E]];

% allowed for H|T is H only, and all that is allowed for T,
% as well as that same set with H included.
allowed([H|T]) -> 
	L = allowed(T),
	L ++ [[H]] ++ lists:map(fun(E) -> [H|E] end, L).