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backend/node_modules/stylehacks/src/isMixin.js

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+'use strict';
+/**
+ * @param {import('postcss').Rule} node
+ * @return {boolean}
+ */
+module.exports = function isMixin(node) {
+  const { selector } = node;
+
+  // If the selector ends with a ':' it is likely a part of a custom mixin.
+  if (!selector || selector[selector.length - 1] === ':') {
+    return true;
+  }
+
+  return false;
+};