@Override
  public ASTNode findChildByRole(int role) {
    LOG.assertTrue(ChildRole.isUnique(role));
    switch(role){
      case ChildRole.OPERAND:
        return getFirstChildNode();

      case ChildRole.OPERATION_SIGN:
        return getLastChildNode();

      default:
        return null;
    }
  }
@Override
  public ASTNode findChildByRole(int role)