setup

# File lib/set.rb, line 446
    def setup   # :nodoc:
      @@setup and return

      module_eval {
        # a hack to shut up warning
        alias old_init initialize
        remove_method :old_init
      }
      begin
        require 'rbtree'

        module_eval %{
          def initialize(*args, &block)
            @hash = RBTree.new
            super
          end
        }
      rescue LoadError
        module_eval %{
          def initialize(*args, &block)
            @keys = nil
            super
          end

          def clear
            @keys = nil
            super
          end

          def replace(enum)
            @keys = nil
            super
          end

          def add(o)
            @keys = nil
            @hash[o] = true
            self
          end
          alias << add

          def delete(o)
            @keys = nil
            @hash.delete(o)
            self
          end

          def delete_if
            n = @hash.size
            super
            @keys = nil if @hash.size != n
            self
          end

          def merge(enum)
            @keys = nil
            super
          end

          def each
            block_given? or return enum_for(__method__)
            to_a.each { |o| yield(o) }
            self
          end

          def to_a
            (@keys = @hash.keys).sort! unless @keys
            @keys
          end
        }
      end

      @@setup = true
    end