* This module provides storage for a set of items of the same type. * * The API offers add(), remove(), in (returning a bool for membership), * clear, empty, length, iteration, dup, and toString. It also supports * set union with & and &= and set intersection with | and |= as well as * == and !=. * * Sets cast to bool (empty giving false, nonempty giving true), so can be * used in conditionals. * * See the unittest block at the end for examples. * * Authors: Mark Summerfield, mark@qtrac.eu * License: Apache 2.0 * Copyright © 2020 Mark Summerfield. All rights reserved.