How to inject element registry to custom rule provider?

Currently I’ve got this setup:

import RuleProvider from 'diagram-js/lib/features/rules/RuleProvider';
import { ElementType, EventBus, Shape } from '../models/bpmn';

export class CustomRules extends RuleProvider {
  [x: string]: any;
  constructor(eventBus: EventBus) {
  static inject$ = ['eventBus'];

  addRule(actions: string | string[], priority: number, cb: Function) {
    if (typeof actions === 'string') {
      actions = [actions];
    actions.forEach(action => {
      this.canExecute(action, priority, (context, action, event) => {
        return cb(context);
      }, true)

  init() {
     // my rules

But I would like to inject element registry to get information about the elements on the diagram to add some extra rules. For example, I want to restrict users from adding start event from the pallette if it already exists on the diagram.
How do I do that?

Just adding elementRegistry to the constructor and to inject$ array didn’t work.

Turns out I made a typo in injecting. It should have been $inject instead of inject$.