how to find out the first occurence of a node with a c

I'm trying to find out if a node with certain string value is occuring
first time. This is the XML snippet:
.....
<letter>A</letter>
<letter>D</letter>
<letter>C</letter>
<letter>D</letter>
......

I want to test if;
   <xsl:if test="starts-with($mod-init,'D')[1]"> or  <xsl:if
test="starts-with($mod-init,'D')[2]"> and so forth, but this code does
not work. What should I do?

Thanks for the help.

Ci 
	 


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