code: function enter(states, opts) {
      return this.concurrent ?
        enterConcurrent.call(this, states, opts) :
        enterClustered.call(this, states, opts);
    }
docs: Internal: Enters the receiver state. The actual entering logic is in the `enterClustered` and `enterConcurrent` methods.  states - An array of destination states. opts   - The options passed to `goto`.  Returns the receiver.