code: protected static String receipientsAsCommaSeperated(final List<BigInteger> recipients) {
          // Note: I didn't opt for using apache commons to reduce the number of includes
          // Let me know if you want to have this changed...
          if (recipients == null) {
              return null;
          }
          if (recipients.size() == 0) {
              return EMPTY;
          }
          final StringBuilder sb = new StringBuilder(recipients.size() * 10);
          for (final BigInteger s : recipients) {
              sb.append(s.toString()).append(",");
          }
          return sb.substring(0, sb.length() - 1);
      }
docs: generates a string from  List<BigInteger>
  
  @param recipients
  @return