x1 is a connective question, requesting the connective between x2 (du'u) and x3 (du'u) in that order, assuming that they are true.
This word is to "ji" approzimately as "kanxe" is to "je". See also: vlina, kanxe, 2 is logically equivalent to x3; x2 being true is sufficient and necessary for guaranteeing the truth of x3; x2 iff x3").">tsida, ribga.