code: public StrBuilder appendSeparator(final String separator, final int loopIndex) {
          if (separator != null && loopIndex > 0) {
              append(separator);
          }
          return this;
      }
docs: Appends a separator to the builder if the loop index is greater than zero.
  Appending a null separator will have no effect.
  The separator is appended using {@link #append(String)}.
  <p>
  This method is useful for adding a separator each time around the
  loop except the first.
  </p>
  <pre>
  for (int i = 0; i &lt; list.size(); i++) {
  appendSeparator(",", i);
  append(list.get(i));
  }
  </pre>
  Note that for this simple example, you should use
  {@link #appendWithSeparators(Iterable, String)}.
  
  @param separator  the separator to use, null means no separator
  @param loopIndex  the loop index
  @return this, to enable chaining
  @since 2.3