class MySet(*set) {

    method init {
        var elems = set
        set = Hash()
        elems.each { |e| self += e }
    }

    method +(elem) {
        set{elem} = elem
        self
    }

    method del(elem) {
        set.delete(elem)
    }

    method has(elem) {
        set.has_key(elem)
    }

    method ∪(MySet that) {
        MySet(set.values..., that.values...)
    }

    method ∩(MySet that) {
        MySet(set.keys.grep{ |k| k ∈ that } \
                    .map { |k| set{k} }...)
    }

    method ∖(MySet that) {
        MySet(set.keys.grep{|k| !(k ∈ that) } \
                    .map {|k| set{k} }...)
    }

    method ^(MySet that) {
        var d = ((self ∖ that) ∪ (that ∖ self))
        MySet(d.values...)
    }

    method count { set.len }

    method ≡(MySet that) {
        (self ∖ that -> count.is_zero) && (that ∖ self -> count.is_zero)
    }

    method values { set.values }

    method ⊆(MySet that) {
        that.set.keys.each { |k|
            k ∈ self || return false
        }
        return true
    }

    method to_s {
        "Set{" + set.values.map{|e| "#{e}"}.sort.join(', ') + "}"
    }
}

class Object {
    method ∈(MySet set) {
        set.has(self)
    }
}