code: function Precondition(properties) {
                      if (properties)
                          for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                              if (properties[keys[i]] != null)
                                  this[keys[i]] = properties[keys[i]];
                  }
docs: Properties of a Precondition.
  @memberof google.firestore.v1
  @interface IPrecondition
  @property {boolean|null} [exists] Precondition exists
  @property {google.protobuf.ITimestamp|null} [updateTime] Precondition updateTime
   
  Constructs a new Precondition.
  @memberof google.firestore.v1
  @classdesc Represents a Precondition.
  @implements IPrecondition
  @constructor
  @param {google.firestore.v1.IPrecondition=} [properties] Properties to set