在xslt中检查节点是否存在

I am new to xslt, Though I manage to work on it. But I struck with one case for which I need a help from expert. I am trying to finding the node which is Exists or not. I already search this same question, in which I didn't get the expected answer. I am having a node, MainNode->SubNode1->SubNode2->SunNode3. If I am checking like belowI am new to xslt, Though I manage to work on it