protected List<Expression> defaultMerge(Expression left, Expression right) {
    List<Expression> listExpressions = new ArrayList<>();
    if (left != null) {
      listExpressions.add(left);
    }
    listExpressions.add(this);
    if (right != null) {
      listExpressions.add(right);
    }

    return listExpressions;
  }
Base merge implementation.
No merge is done, simply returns the left expression, this and the right expression.

@param left   the expression to the left of this expression
@param right  the expression to the right of this expression

@return a list containing the un-merged left expression, this and right expression
ambari-server/src/main/java/org/apache/ambari/server/api/predicate/expressions/AbstractExpression.java