code: public MoneyFormatterBuilder appendSigned(
              MoneyFormatter whenPositiveOrZero, MoneyFormatter whenNegative) {
          return appendSigned(whenPositiveOrZero, whenPositiveOrZero, whenNegative);
      }
docs: Appends the specified formatters, one used when the amount is positive,
  and one when the amount is negative.
  <p>
  When printing, the amount is queried and the appropriate formatter is used.
  <p>
  When parsing, each formatter is tried, with the longest successful match,
  or the first match if multiple are successful. If the negative parser is
  matched, the amount returned will be negative no matter what amount is parsed.
  <p>
  A typical use case for this would be to produce a format like
  '{@code ($123)}' for negative amounts and '{@code $123}' for positive amounts.
  <p>
  In order to use this method, it may be necessary to output an unsigned amount.
  This can be achieved using {@link #appendAmount(MoneyAmountStyle)} and
  {@link MoneyAmountStyle#withAbsValue(boolean)}.
  
  @param whenPositiveOrZero  the formatter to use when the amount is positive or zero
  @param whenNegative  the formatter to use when the amount is negative
  @return this for chaining, never null