Extend all and leading/trailing combinators

[SomMeri]

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;
}