# File lib/mail/parsers/content_transfer_encoding.rb, line 101
    def _nt_encoding
      start_index = index
      if node_cache[:encoding].has_key?(index)
        cached = node_cache[:encoding][index]
        if cached
          cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
          @index = cached.interval.end
        end
        return cached
      end

      i0 = index
      i1, s1 = index, []
      r2 = _nt_ietf_token
      s1 << r2
      if r2
        if has_terminal?("s", false, index)
          r4 = instantiate_node(SyntaxNode,input, index...(index + 1))
          @index += 1
        else
          terminal_parse_failure("s")
          r4 = nil
        end
        if r4
          r3 = r4
        else
          r3 = instantiate_node(SyntaxNode,input, index...index)
        end
        s1 << r3
      end
      if s1.last
        r1 = instantiate_node(SyntaxNode,input, i1...index, s1)
        r1.extend(Encoding0)
        r1.extend(Encoding1)
      else
        @index = i1
        r1 = nil
      end
      if r1
        r0 = r1
      else
        r5 = _nt_custom_x_token
        if r5
          r0 = r5
        else
          @index = i0
          r0 = nil
        end
      end

      node_cache[:encoding][start_index] = r0

      r0
    end