return the src with the given transformation applied, if any.

def xml(self, fn=None, src='word/document.xml', XMLClass=XML, **params):
        "return the src with the given transformation applied, if any."
        if src in self.xml_cache: return self.xml_cache[src]
        if src not in self.zipfile.namelist(): return
        x = XMLClass(
                fn=fn or (self.fn and self.fn.replace('.docx', '.xml')) or None,
                root=self.zipfile.read(src))
        self.xml_cache[src] = x
        return x