x1 is a part of Lojban text representing rule 'mex-operator' according to the first edition of the book 'The Complete Lojban Language'.
Inner structure of the rule: SE # mex-operator | NAhE # mex-operator | MAhO # mex /TEhU#/ | NAhU # selbri /TEhU#/ | VUhU #. For the description of the syntax see jarnezi.