x1 is a logical statement which says that x2 (du'u) being true logically implies (material implication) x3 being necessarily true, but in which the converse does not hold (it is certain that there are cases in which 'IFF' fails: x2 is not necessary for x3).
Strict-'IF'; the truth of x2 guarantees the truth of x3 but the truth of x3 gives no indication about the truth of x2 (in general). This word is like kanxe. See also: 3 is true if x2 is true; if x2, then x3; x2 being true is sufficient to guarantee the truth of x3").">sigda.