x1 is a part of Lojban text representing rule 'tanru-unit-2' according to the first edition of the book 'The Complete Lojban Language'.
Inner structure of the rule: BRIVLA # | GOhA RAhO? # | KE # selbri-3 /KEhE#/ | ME # sumti /MEhU#/ (MOI #)? | (number | lerfu-string) MOI # | NUhA # mex-operator | SE # tanru-unit-2 | JAI # tag? tanru-unit-2 | any-word (ZEI any-word)+ | NAhE # tanru-unit-2 | NU NAI? # (joik-jek NU NAI? #)* subsentence /KEI#/. For the description of the syntax see jarnezi.