# File lib/openid/message.rb, line 510
    def add(namespace_uri)
      # see if this namepace is already mapped to an alias
      _alias = @namespace_to_alias[namespace_uri]
      return _alias if _alias

      # Fall back to generating a numberical alias
      i = 0
      while true
        _alias = 'ext' + i.to_s
        begin
          add_alias(namespace_uri, _alias)
        rescue IndexError
          i += 1
        else
          return _alias
        end
      end

      raise StandardError, 'Unreachable'
    end