# File lib/mail/parsers/content_type.rb, line 754
    def _nt_sub_type
      start_index = index
      if node_cache[:sub_type].has_key?(index)
        cached = node_cache[:sub_type][index]
        if cached
          cached = SyntaxNode.new(input, index...(index + 1)) if cached == true
          @index = cached.interval.end
        end
        return cached
      end

      i0 = index
      r1 = _nt_extension_token
      if r1
        r0 = r1
      else
        r2 = _nt_iana_token
        if r2
          r0 = r2
        else
          @index = i0
          r0 = nil
        end
      end

      node_cache[:sub_type][start_index] = r0

      r0
    end