code: public BigMoney plus(BigDecimal amountToAdd) {
          MoneyUtils.checkNotNull(amountToAdd, "Amount must not be null");
          if (amountToAdd.compareTo(BigDecimal.ZERO) == 0) {
              return this;
          }
          BigDecimal newAmount = amount.add(amountToAdd);
          return BigMoney.of(currency, newAmount);
      }
docs: Returns a copy of this monetary value with the amount added.
  <p>
  This adds the specified amount to this monetary amount, returning a new object.
  <p>
  No precision is lost in the result.
  The scale of the result will be the maximum of the two scales.
  For example, 'USD 25.95' plus '3.021' gives 'USD 28.971'.
  <p>
  This instance is immutable and unaffected by this method.
  
  @param amountToAdd  the monetary value to add, not null
  @return the new instance with the input amount added, never null