x1 is a part of Lojban text representing rule 'operand-3' according to the first edition of the book 'The Complete Lojban Language'.
Inner structure of the rule: quantifier | lerfu-string /BOI#/ | NIhE # selbri /TEhU#/ | MOhE # sumti /TEhU#/ | JOhI # mex-2+ /TEhU#/ | gek operand gik operand-3 | (LAhE # | NAhE BO #) operand /LUhU#/. For the description of the syntax see jarnezi.