A static node reprenting an `@extend` directive.
@see Sass::Tree
The parsed selector after interpolation has been resolved. Only set once {Tree::Visitors::Perform} has been run.
@return [Selector::CommaSequence]
The CSS selector to extend, interspersed with {Sass::Script::Node}s representing `#{}`-interpolation.
@return [Array<String, Sass::Script::Node>]
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