code: public DateTimeFormatterBuilder append(DateTimePrinter printer) {
          checkPrinter(printer);
          return append0(DateTimePrinterInternalPrinter.of(printer), null);
      }
docs: Appends just a printer. With no matching parser, a parser cannot be
  built from this DateTimeFormatterBuilder.
  <p>
  The printer interface is part of the low-level part of the formatting API.
  Normally, instances are extracted from another formatter.
  Note however that any formatter specific information, such as the locale,
  time-zone, chronology, offset parsing or pivot/default year, will not be
  extracted by this method.
  
  @param printer  the printer to add, not null
  @return this DateTimeFormatterBuilder, for chaining
  @throws IllegalArgumentException if printer is null or of an invalid type