mathematical/mekso binary operator: the zero/identity-element/(primitive (-))constant operator; outputs the identity-element of structure A (contextless default: the additive group of integers) regardless of the input value of B (except blank or ill-defined values)

According to the contextless default, the (contextless default) output will be identically 0. B can be literally anything imaginable (no specific type is required), so long as B is non-empty (not blank) and is well-defined; if these conditions are satisfied, the identity-element of structure A is output; else, this function is undefined. Notice that in Lojban, a blank must be input explicitly; otherwise xo'o will fill the argument implicitly. By skipping arguments in this manner (assuming a known default and using implicit elliptical argument-filling features of Lojban), this operator can be reduced to 0-arity. Note that this operator is different from multiplying by 0. See also: bai'ei, ci'o'au.

- ci'o'au
- mekso operator (binary): projection function; the Bth term/entry ("element") of tuple A