Functional sets in Javascript

gistfile1.js

function empty() {
    return function() {
        return false;
    };
}

function singleton(e0) {
    return function(e) {
        return e0 === e;
    };
}

function contains(s, e) {
    return s(e);
}

function union(s1, s2) {
    return function(e) {
        return contains(s1, e) || contains(s2, e);
    };
}

function intersect(s1, s2) {
    return function(e) {
        return contains(s1, e) && contains(s2, e);
    };
}

function difference(s1, s2) {
    return function(e) {
        return contains(s1, e) && !contains(s2, e);
    };
}

function filter(s, pred) {
    return function(e) {
        return pred(e) && contains(s, e);
    };
}

function add(s, e) {
    return union(s, singleton(e));
}

function remove(s, e) {
    return difference(s, singleton(e));
}

function toSet(l) {
    var acc = empty();
    for (var i = 0; i < l.length; i++) {
        acc = union(acc, singleton(l[i]));
    }
    return acc;
}

function setOf() {
    var args = Array.prototype.slice.call(arguments);
    return toSet(args);
}