Get position of element
===

Suppose I have a xml 

    <Root>
    <TB>
    ..
    </TB>
    <TB>
    ..
    
    </TB>
    <TB>
    ..
    <DIACT>2</DIACT>
    </TB>
    <TB>
    ...
    <DIACT>3</DIACT>
    </TB>
    <TB>
    ----
    <DIACT>4</DIACT>
    ---
    </TB>
    </Root>

I want to get the **position** of first TB node where DIACT is not null or DIACT node exist, using xslt mapping. Like in this case postion should give me output as 3. As 3rd TB node is the first one with DIACT not null.