js: recursive 2 set operations

set.js

function diff(a, b) {
	return _operation(a, b, [], diff, true)
}

function union(a, b) {
	return _operation(a, b, b, union, true)
}

function intersect(a, b) {
	return _operation(a, b, [], intersect)
}

function xor(a, b) {
	return union(diff(a, b), diff(b, a))
}

function equal(a, b) {
	return !xor(a, b).length
}

function subset(a, b) {
	return equal(intersect(a, b), a)
}

function membership(w, o) {
	if (!o.length) return false
	else return w == o[0] ? true : membership(w, o.slice(1))
}

function _operation(a, b, r, f, p) {
	if (!a.length) return r
	else return membership(a[0], b) == !!p ? f(a.slice(1), b) : [a[0]].concat(f(a.slice(1), b))
}