deleteMapKey(path, key) {
    const objectId = path.length === 0 ? '_root' : path[path.length - 1].objectId
    const object = this.getObject(objectId)

    if (object[key] !== undefined) {
      const pred = getPred(object, key)
      this.addOp({action: 'del', obj: objectId, key, insert: false, pred})
      this.applyAtPath(path, subpatch => {
        subpatch.props[key] = {}
      })
    }
  }
Updates the map object at path `path`, deleting the property `key`.