add_alias (OpenID::NamespaceMap)

    # File lib/openid/message.rb, line 461
        def add_alias(namespace_uri, desired_alias)
          # Check that desired_alias is not an openid protocol field as
          # per the spec.
          Util.assert(!OPENID_PROTOCOL_FIELDS.include?(desired_alias),
                 "#{desired_alias} is not an allowed namespace alias")

          # check that there is not a namespace already defined for the
          # desired alias
          current_namespace_uri = @alias_to_namespace.fetch(desired_alias, nil)
          if current_namespace_uri and current_namespace_uri != namespace_uri
            raise IndexError, "Cannot map #{namespace_uri} to alias #{desired_alias}. #{current_namespace_uri} is already mapped to alias #{desired_alias}"
          end

          # Check that desired_alias does not contain a period as per the
          # spec.
          if desired_alias.is_a?(String)
              Util.assert(desired_alias.index('.').nil?,
                     "#{desired_alias} must not contain a dot")
          end

          # check that there is not already a (different) alias for this
          # namespace URI.
          _alias = @namespace_to_alias[namespace_uri]
          if _alias and _alias != desired_alias
            raise IndexError, "Cannot map #{namespace_uri} to alias #{desired_alias}. It is already mapped to alias #{_alias}"
          end

          @alias_to_namespace[desired_alias] = namespace_uri
          @namespace_to_alias[namespace_uri] = desired_alias
          return desired_alias
        end