return {
	make: make
};

function make(type) {
	if(type == undefined) {
		var id = function(x) {return x;};
		type = {toKey: id, ofKey: id};
	}

	function Set(s) {
		this.elements = {};
		if(Array.isArray(s)) {
			s.forEach(this.add.bind(this));
		} else if(s != undefined) {
			this.add(s);
		}
	}

	Set.prototype.add = function(e) { this.elements[type.toKey(e)] = true; };
	Set.prototype.remove = function(e) { delete this.elements[type.toKey(e)]; };
	Set.prototype.contains = function(e) { return !!this.elements[type.toKey(e)]; };
	Set.prototype.size = function() { return Object.keys(this.elements).length; };

	Set.prototype.toList = function() {
		var result = [];
		for(var k in this.elements) {
			result.push(type.ofKey(k));
		}
		return Array.from(result).sort();
	}

	Set.prototype.iter = function(f) { this.toList().forEach(f); };
	Set.prototype.map = function(f) { return this.toList().map(f); };

	Set.prototype.equals = function(set) {
		var sorted = [this.toList(), set.toList()];
		return sorted[0].length == sorted[1].length &&
			sorted[0].every(function(x) {return x == sorted[1].shift();});
	}

	Set.prototype.subset = function(predicate) {
		var newSet = new Set();
		this.iter(function(e) {
			if(predicate(e)) {
				newSet.add(e);
			}
		});
		return newSet;
	};

	Set.prototype.difference = function(set) {
		return this.subset(function(e) {return !set.contains(e);});
	}

	Set.prototype.intersection = function(set) {
		return this.subset(function(e) {return set.contains(e);});
	}

	Set.union = function(sets) {
		var newSet = new Set();
		for(var i = 0; i < sets.length; i++) {
			sets[i].iter(newSet.add.bind(newSet));
		}
		return newSet;
	}

	return Set;
}