stairs.rb

      def on_call_lang(input, arg, &block)
        lang_var = unique_literal('lang')
        node     = unique_literal('node')
        found    = unique_literal('found')
        xml_lang = unique_literal('xml_lang')
        matched  = unique_literal('matched')

        conversion = literal(Conversion)

        ast = lang_var.assign(try_match_first_node(arg, input))
          .followed_by(lang_var.assign(conversion.to_string(lang_var)))
          .followed_by(matched.assign(self.false))
          .followed_by(node.assign(input))
          .followed_by(xml_lang.assign(string('xml:lang')))
          .followed_by do
            node.respond_to?(symbol(:attribute)).while_true do
              found.assign(node.get(xml_lang))
                .followed_by do
                  found.if_true do
                    found.eq(lang_var)
                      .if_true do
                        if block_given?
                          yield
                        else
                          matched.assign(self.true)
                            .followed_by(send_message(:break))
                        end
                      end
                      .else { send_message(:break) }
                  end
                end
                .followed_by(node.assign(node.parent))
            end
          end

        block_given? ? ast : ast.followed_by(matched)
      end