code: public function offsetUnset($offset)
      {
          $offset = $this->calculateOffset($offset);
          if (isset($this->_children[$offset])) {
              array_splice($this->_children, $offset, 1);
          }
          $this->updateCollectionOffsets();
      }
docs: Removes a child token at the specified offset.
  This method is required by the SPL [[\ArrayAccess]] interface.
  It is implicitly called when you use something like `unset($token[$offset])`.
  @param int $offset child token offset.