Example of how to pass selectors to a mixin in LESS

fade-toggle.less

.fade-toggle (@openSelector: ~'.open'; @inTransitionClassName: ~'transition-helper'; @openRight: 0; @zIndex: 1) {
  .transition(opacity .25s ease-in-out);
  position: absolute;
  left: -9999em;
  z-index: @zIndex;
  opacity: 0;

  @rls: {
    left: 0;
    right: @openRight;
  };

  &.@{inTransitionClassName} {
    @rls();
  }

  .loop(@counter) when (@counter > 0) {
    @selector: extract(@openSelector, @counter);
    @{selector} > & {
      @rls();
      opacity: 1;
    }
    .loop((@counter - 1));
  }

  .loop(length(@openSelector));
}

.subnav {
  .fade-toggle(~'li:hover', ~'.open'; @openRight: auto);
}