unify method
Returns a SelectorList that matches only elements that are matched by
both this and other
.
If no such list can be produced, returns null
.
Implementation
SelectorList? unify(SelectorList other) {
var contents = [
for (var complex1 in components)
for (var complex2 in other.components)
...?unifyComplex([complex1, complex2], complex1.span)
];
return contents.isEmpty ? null : SelectorList(contents, span);
}