It appears that the repository has undergone several changes and renamings:

backend/node_modules/postcss/lib/rule.js

@@ -1,27 +0,0 @@
-'use strict'
-
-let Container = require('./container')
-let list = require('./list')
-
-class Rule extends Container {
-  get selectors() {
-    return list.comma(this.selector)
-  }
-
-  set selectors(values) {
-    let match = this.selector ? this.selector.match(/,\s*/) : null
-    let sep = match ? match[0] : ',' + this.raw('between', 'beforeOpen')
-    this.selector = values.join(sep)
-  }
-
-  constructor(defaults) {
-    super(defaults)
-    this.type = 'rule'
-    if (!this.nodes) this.nodes = []
-  }
-}
-
-module.exports = Rule
-Rule.default = Rule
-
-Container.registerRule(Rule)