"Additive" property values with LESS mixin - less

Question

I'm looking for a feature that may or may not be available in LESS.
I have a mixin that adds a "glow" with box-shadow, which I use on various elements - buttons, inputs etc.
.glow() {
box-shadow: 0 0 5px skyBlue;
}
What I'm looking for is a way to make the mixin add the box-shadow as a new comma-seperated value if the element already has a box-shadow.
So, I want this:
.button {
box-shadow: inset 0 5px 10px black;
.glow();
}
To compile to this:
.button {
box-shadow: inset 0 5px 10px black, 0 0 5px skyBlue;
}
I think I recall seeing a similar feature in SASS, but I can't find it anywhere now.
Does this feature exist in LESS, or is there some alternative way to achieve a similar result?

Answer 1

The feature you're looking for is merge. You'd do this:
.glow() {
box-shadow+: 0 0 5px skyBlue;
}
.button {
box-shadow+: inset 0 5px 10px black;
.glow();
}
Note that both rulesets need to use the + syntax for it to work.
Or, you could declare the glow rule as a variable:
#glow: 0 0 5px skyBlue;
.button {
box-shadow: inset 0 5px 10px black, #glow;
}