function emptySet()         return { }  end
function insert(set, item)  set[item] = true  end
function remove(set, item)  set[item] = nil  end
function member(set, item)  return set[item]  end
function size(set)
	local result = 0
	for _ in pairs(set) do result = result + 1 end
	return result
end
function fromTable(tbl)