code: public static String join(long[] x, String separator) {
          StringBuilder result = new StringBuilder();
          for (int i = 0; i < x.length; ++i) {
              result.append(str(x[i]));
              if (i + 1 < x.length) {
                  result.append(separator);
              }
          }
          return result.toString();
      }
docs: Joins an array of elements by the given separator into a single string. The
  elements are represented as strings utilizing the Op.str() method on each
  element
  
  @param x is the array of elements
  @param separator is the string to add between the elements
  @return a single string joining the array of elements by the given
  separator