protected function missingSelectors()
    {
        foreach ($this->extends as $extend) {
            if (isset($extend[3])) {
                continue;
            }

            list($target, $origin, $block) = $extend;

            // ignore if !optional
            if ($block[2]) {
                continue;
            }

            $target = implode(' ', $target);
            $origin = $this->collapseSelectors($origin);

            $this->sourceLine = $block[Parser::SOURCE_LINE];
            $this->throwError("\"$origin\" failed to @extend \"$target\". The selector \"$target\" was not found.");
        }
    }