local function equal(a, b)
	for k in pairs(a) do
		if not b[k] then
			return false
		end
	end
	for k in pairs(b) do
		if not a[k] then
			return false
		end
	end
	return true
end

local function min(s)
	local a
	for b in pairs(s) do
		if not a or b < a then
			a = b
		end
	end
	return a
end

local function max(s)
	local a
	for b in pairs(s) do
		if not a or a < b then
			a = b
		end
	end
	return a
end

local function merge(to, from)
	for k in pairs(from) do
		to[k] = true
	end
end

return {equal = equal, min = min, max = max, merge = merge}