x1 is a part of Lojban text representing rule 'free' according to the first edition of the book 'The Complete Lojban Language'.
Inner structure of the rule: SEI # (terms (CU #)?)? selbri /SEhU/ | SOI # sumti sumti? /SEhU/ | vocative (relative-clauses)? selbri (relative-clauses)? /DOhU/ | vocative (relative-clauses)? CMENE+ # (relative-clauses)? /DOhU/ | vocative sumti? /DOhU/ | (number | lerfu-string) MAI | TO text /TOI/ | XI # (number | lerfu-string) /BOI/ | XI # VEI # mex /VEhO/. For the description of the syntax see jarnezi.