# File lib/mail/parsers/rfc2822.rb, line 3103
    def _nt_day_name
      start_index = index
      if node_cache[:day_name].has_key?(index)
        cached = node_cache[:day_name][index]
        if cached
          cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
          @index = cached.interval.end
        end
        return cached
      end

      i0 = index
      if has_terminal?("Mon", false, index)
        r1 = instantiate_node(SyntaxNode,input, index...(index + 3))
        @index += 3
      else
        terminal_parse_failure("Mon")
        r1 = nil
      end
      if r1
        r0 = r1
      else
        if has_terminal?("Tue", false, index)
          r2 = instantiate_node(SyntaxNode,input, index...(index + 3))
          @index += 3
        else
          terminal_parse_failure("Tue")
          r2 = nil
        end
        if r2
          r0 = r2
        else
          if has_terminal?("Wed", false, index)
            r3 = instantiate_node(SyntaxNode,input, index...(index + 3))
            @index += 3
          else
            terminal_parse_failure("Wed")
            r3 = nil
          end
          if r3
            r0 = r3
          else
            if has_terminal?("Thu", false, index)
              r4 = instantiate_node(SyntaxNode,input, index...(index + 3))
              @index += 3
            else
              terminal_parse_failure("Thu")
              r4 = nil
            end
            if r4
              r0 = r4
            else
              if has_terminal?("Fri", false, index)
                r5 = instantiate_node(SyntaxNode,input, index...(index + 3))
                @index += 3
              else
                terminal_parse_failure("Fri")
                r5 = nil
              end
              if r5
                r0 = r5
              else
                if has_terminal?("Sat", false, index)
                  r6 = instantiate_node(SyntaxNode,input, index...(index + 3))
                  @index += 3
                else
                  terminal_parse_failure("Sat")
                  r6 = nil
                end
                if r6
                  r0 = r6
                else
                  if has_terminal?("Sun", false, index)
                    r7 = instantiate_node(SyntaxNode,input, index...(index + 3))
                    @index += 3
                  else
                    terminal_parse_failure("Sun")
                    r7 = nil
                  end
                  if r7
                    r0 = r7
                  else
                    @index = i0
                    r0 = nil
                  end
                end
              end
            end
          end
        end
      end

      node_cache[:day_name][start_index] = r0

      r0
    end