protected function guardAgainstImpossibleMove() {
    if ( !$this->node->exists )
      throw new MoveNotPossibleException('A new node cannot be moved.');

    if ( array_search($this->position, array('child', 'left', 'right', 'root')) === FALSE )
      throw new MoveNotPossibleException("Position should be one of ['child', 'left', 'right'] but is {$this->position}.");

    if ( !$this->promotingToRoot() ) {
      if ( is_null($this->target) ) {
        if ( $this->position === 'left' || $this->position === 'right' )
          throw new MoveNotPossibleException("Could not resolve target node. This node cannot move any further to the {$this->position}.");
        else
          throw new MoveNotPossibleException('Could not resolve target node.');
      }

      if ( $this->node->equals($this->target) )
        throw new MoveNotPossibleException('A node cannot be moved to itself.');

      if ( $this->target->insideSubtree($this->node) )
        throw new MoveNotPossibleException('A node cannot be moved to a descendant of itself (inside moved tree).');

      if ( !$this->node->inSameScope($this->target) )
        throw new MoveNotPossibleException('A node cannot be moved to a different scope.');
    }
  }