# File lib/xsd/ns.rb, line 58
  def assign(ns, tag = nil)
    if tag == ''
      if ns.empty?
        @default_namespace = nil
      else
        @default_namespace = ns
      end
      tag
    else
      @assigner ||= Assigner.new(default_known_tag)
      tag ||= @assigner.assign(ns)
      @ns2tag[ns] = tag
      @tag2ns[tag] = ns
      tag
    end
  end