code: public void setMultipleSeparator(final String multipleSeparator) {
          if (multipleSeparator != null)
              attrMixin.setAttribute(MULTIPLE_SEPARATOR, multipleSeparator);
          else
              attrMixin.removeAttribute(MULTIPLE_SEPARATOR);
      }
docs: Sets the character displayed in the button that separates
  selected options.<br>
  <br>
  Defaults to <code>, </code>.
  
  @param multipleSeparator