def in_namespace(name)
      name ||= generate_name
      @scope = Scope.new(name, @scope)
      ns = NameSpace.new(self, @scope)
      yield(ns)
      ns
    ensure
      @scope = @scope.tail
    end
def in_namespace(name)
      name ||= generate_name
      @scope = Scope.new(name, @scope)
      ns = NameSpace.new(self, @scope)
      yield(ns)
      ns
    ensure
      @scope = @scope.tail
    end
Evaluate the block in a nested namespace named +name+.  Create
 an anonymous namespace if +name+ is nil.