code: public static MonetaryAmount sum(MonetaryAmount a, MonetaryAmount b){
          MoneyUtils.checkAmountParameter(Objects.requireNonNull(a), Objects.requireNonNull(b.getCurrency()));
          return a.add(b);
      }
docs: Adds two monetary together
  @param a the first operand
  @param b the second operand
  @return the sum of {@code a} and {@code b}
  @throws NullPointerException if a o b be null
  @throws MonetaryException    if a and b have different currency