keyframe-name = pulse
$val = 20

@keyframes { $keyframe-name }
  for i in 0..5
    {20% * i}
      opacity (i / $val)

@keyframes auto-color
  0%
    color red
  50%
    color blue
  100%
    color yellow

@keyframes foo
  from
    color: black
  to
    color: white