Enable &:extend() to take a class as a variable

Feature Request

It would be very powerful if :extend could be passed a class name as a variable.

e.g. something like:

&:extend(@{extending-class-string});

or:

&:extend(@extending-class);

Use Cases:

(tl;dr this would be brilliant for making mixins reusable and for grid systems)

If you want to extend a class with numbered additions it is possible, as long as you create a mixin for every class you want to extend.

for example:

.dec-loop (@length) {
  @i: @length;

  .extended-class {
    float: left;
  }

  .loop (@i) when (@i > 0) {   
    @loop-class: ~".dec-@{i}";   
    @{loop-class} {
      &:extend(.extended-class);
    }    
    .loop(@i - 1);
  }
  .loop(@i);  
}
.dec-loop(4);

will create:

.extended-class,
.extended-class-4,
.extended-class-3,
.extended-class-2,
.extended-class-1 {
  float: left;
}

However the name of the class being extended is hard coded into the mixin, requiring a new mixin for every class you want to extend in this way. But imagine if this worked:

.inc-loop (@extending-class-name; @total) {
  @i: 1;
  @extending-class-string: ~".@{extending-class-name}";

  .loop (@i) when (@i =< @total) {   
    @loop-class: ~".@{extending-class-name}-@{i}";   
    @{loop-class} {     
      &:extend(@{extending-class-string}); // feature request
    }    
    .loop(@i + 1);
  }
  .loop(@i);  
}

.extended-class {
  float: left;
}
.inc-loop(extended-class, 8);

or even better, if this worked!:

.inc-loop (@extending-class-name; @total) {
  @i: 1;
  @extending-class-string: ~".@{extending-class-name}";

  .@{extending-class-name} { // another feature request?
    float: left;
  }

  .loop (@i) when (@i =< @total) {   
    @loop-class: ~".@{extending-class-name}-@{i}";   
    @{loop-class} {
      &:extend(@{extending-class-string}); // feature request
    }    
    .loop(@i + 1);
  }
  .loop(@i);  
}

.inc-loop(extended-class, 8);

Because this would allow us to have a SINGLE mixin, that could create and extend many different classes. Which might be just what you want if you are working on a grid layout system.

P.S.
I suspect that:

.@{extending-class-name}{}

may not work for quite a different reason. Somehow it does not create a class that can be extended, however this probably ought to be a separate Feature Request or Bug Issue.

Start with the existing :extend entry point and the variable/interpolation behavior shown in the examples. Define expected output for a variable-provided class, including whether the separate dynamic class-name request is in scope, then verify that the demonstrated combined selectors work without regressing current :extend behavior.