class set
        declare internal_list

        def set()
                internal_list = list()
        end
        def set(list)
                internal_list = list
        end

        def append(value)
                if not value in internal_list
                        internal_list.append(value)
                end
                return this
        end
        def contains(value)
                return value in internal_list
        end
        def difference(other)
                diff = list()
                for value in this.internal_list
                        diff.append(value)
                end

                for i in range(len(diff) - 1, 0)
                        if diff[i] in other.internal_list
                                diff.remove(i)
                        end
                end

                return new(set, diff)
        end
        def operator=(other)
                for value in other.internal_list
                        if not value in this.internal_list
                                return false
                        end
                end
                return true
        end
        def intersection(other)
                intersect = list()
                for value in this.internal_list
                        if other.contains(value)
                                intersect.append(value)
                        end
                end
                return new(set, intersect)
        end
        def subset(other)
                for value in this.internal_list
                        if not value in other.internal_list
                                return false
                        end
                end
                return true
        end
        def union(other)
                u = list()

                for value in this.internal_list
                        u.append(value)
                end

                for value in other.internal_list
                        if not value in u
                                u.append(value)
                        end
                end

                return new(set, u)
        end

        def toString()
                return str(this.internal_list)
        end
end