logical connective: tanru-internal afterthought biconditional/iff/if-and-only-if.

- jetydu'i
- x
_{1}and x_{2}have the same truth value; either both or none of x_{1}and x_{2}is true - tsida
- x
_{1}is a logical statement of proposition-equivalence/is a biconditional/'IFF' statement saying that x_{2}(du'u) is true if and only if x_{3}is true ("x_{2}is logically equivalent to x_{3}; x_{2}being true is sufficient and necessary for guaranteeing the truth of x_{3}; x_{2}iff x_{3}").