# File lib/xsd/ns.rb, line 43
  def assign(ns, tag = nil)
    if (tag == '')
      @default_namespace = ns
      tag
    else
      @assigner ||= Assigner.new
      tag ||= @assigner.assign(ns)
      @ns2tag[ns] = tag
      @tag2ns[tag] = ns
      tag
    end
  end