$elements: con, slider, masthead;

.masthead {
  /* z-index: 3 */
  z-index: index($elements, masthead);
}

.slider {
  /* z-index: 2 */
  z-index: index($elements, slider);
}

.con {
  /* z-index: 1 */
  z-index: index($elements, con);
}