less.js/test/less/css-guards.less

.light when (lightness(@a) > 50%) {
  color: green;
}
.dark when (lightness(@a) < 50%) {
  color: orange;
}
@a: #ddd;

.see-the {
  @a: #444; // this mirrors what mixins do - they evaluate guards at the point of execution
  .light();
  .dark();
}

.hide-the {
  .light();
  .dark();
}

.multiple-conditions-1 when (@b = 1), (@c = 2), (@d = 3) {
  color: red;
}

.multiple-conditions-2 when (@b = 1), (@c = 2), (@d = 2) {
  color: blue;
}

@b: 2;
@c: 3;
@d: 3;