x1 is a slinku'i word
Note: for obvious reasons, zo will probably not work here. You probably want zoi to quote x1. A slinku'i word form looks like a stage-4 fu'ivla, but would be ambiguous when following certain cmavo, so it is disallowed. For more see http://mw.lojban.org/index.php?title=lo_valslinku%27i_cu_mo. See also valrtosmabru.