I am writing a mixin like this:
@mixin box-shadow($top, $left, $blur, $color, $inset:\"\") {
-webkit-box-shadow:
You can always assign null to your optional arguments. Here is a simple fix
@mixin box-shadow($top, $left, $blur, $color, $inset:null) { //assigning null to inset value makes it optional
-webkit-box-shadow: $top $left $blur $color $inset;
-moz-box-shadow: $top $left $blur $color $inset;
box-shadow: $top $left $blur $color $inset;
}