Extend all and leading/trailing combinators

It is not possible to use leading or trailing combinator inside extend all. Combinators in the middle are compared while:

• leading combinators are ignored
• trailing combinators prevent matching.

a:first b:second  { property: value;}
a:first + b:second  { property: value;}

leading:extend(+ b:second all) {} // + is ignored
trailing:extend(a:first + all) {} // + prevents matching

compiles into:

a:first b:second,
a:first leading { // + was ignored match found 
  property: value;
}
a:first + b:second,
a:first + leading { // trailing is missing here
  property: value;
}

No file or test is named. Start by locating the Less compiler logic for extend-all matching and compare its handling of leading, middle, and trailing combinators with the examples in the issue. Done means leading and trailing combinators are matched consistently and the shown output includes the trailing combinator case.