I want to create a global variable that contains the result tree fragment contained within element <c>, with the following restrictions:
I only want to include the first <y> element that is contained within <c>, no matter where it occurs. There may be no <y> elements present.
I want to exclude all <z> elements that are contained within <c>, no matter where they occur. Again, there may be none present.

I have tried many variations on the select portion of the variable definition. I can filter the immediate children of <c>, OR the second level children, etc., but I can't seem to come up with anything that handles <y> and <z> appearing at any depth in the descendant tree.

Then I tried to just exclude <z> at any level:
<xsl:variable name="set" select="/a/b/c/descendant::*[not(self::z)]" />
but that only excludes <z> elements that are children of <c>.