@Override
  public List<Expression> merge(Expression left, Expression right, int precedence) {
    if (getOperator().getPrecedence() == precedence && getLeftOperand() == null) {
      setLeftOperand(left);
      setRightOperand(right);
      return Collections.singletonList(this);
    } else {
      return defaultMerge(left, right);
    }
  }
Constructor.

@param op  the logical operator of the expression
ambari-server/src/main/java/org/apache/ambari/server/api/predicate/expressions/LogicalExpression.java