BinaryWord from Finset
A subset of 1..n as its characteristic word: letter i is 1 when i is a member.
Not order-preserving: Subsets lists subsets by size, then lex, while BinaryWords(n) through Finset(word) gives them in their characteristic words' order.
Called as CombinatorialMap(x, "BinaryWord"). The head's page has its examples.