Collector variable structure if only one or zero arguments are collected

By collector variables I mean either @arguments variable or any mixin parameter declared with ....

If collector collected multiple values, then it contains a list. However, if it collected only on variable, then it does not contains a list. It contains that one variable. That makes it impossible to differentiate between these two things:

• .mixin(1, 2, 3) // three parameters
• .mixin(1, 2, 3;) // one parameter

Full test case:

.mixin(@first, @rest...) {
  rest-length: length(@rest);
  rest: ~`"@{rest}"`;
}

div-1 {
  .mixin(box-shadow 0.2s linear);
}
div-3 {
  .mixin(box-shadow 0.2s linear, color .4s .2s ease);
}
div-2 {
  .mixin(box-shadow 0.2s linear, color .4s .2s ease, jabba dabba);
}

current output:

div-1 {
  rest-length: 0;
  rest: ;
}
div-2 {
  rest-length: 4;
  rest: [color, 0.4s, 0.2s, ease];
}
div-2 {
  rest-length: 2;
  rest: [color 0.4s 0.2s ease, jabba dabba];
}

Expected output:

div-1 {
  rest-length: 0;
  rest: []; // maybe? 
}
div-2 {
 //here is difference. It is still impossible to tell comma/space difference, but 
 //it is at least possible to know the right number of parameters
  rest-length: 1;
  rest: [[color, 0.4s, 0.2s, ease]]; 
}
div-2 {
  rest-length: 2;
  rest: [[color, 0.4s, 0.2s, ease], [jabba dabba]];
}

Background: I was trying to find out how LessHat could work if we would fix #1939 (pull request #1941). Spaces/commas/argument separator difference is lost in less->js conversion. Which might be ok if would could partially deduce them from structure which is impossible.

LessHat current prints @arguments into string and then uses javascript to parse that string.