mekso operator: in ordered tuple/list/vector/sequence X_{1}, replace the X_{2}th entry with term X_{3} of appropriate type, and leave all other entries untouched (optional: where the index for the very first/leading/header entry is X_{4}).

X_{1} must be some ordered and indexed structure; X_{2} must be an index which specifies an entry/element/term which actually appears in X_{1}; X_{3} is an alternative value of appropriate type which can, and after this operation does, replace the X_{2}th entry/element/term; the counting for indices is according to explicitly specified or implicit cultural convention, or the natural convention for the circumstance, or (when ambiguous or unclear) starts with 1, unless X_{4} specifies otherwise (in which case, it is according to that specification). Exactly the X_{2}th entry is reppaced, and it is replaced with exactly X_{3}. For example: in (A, B, C), A is the X_{4}th entry (default: 1st); bai'i'i((A, B, C), 2, b, 1) = (A, b, C). X_{2} itself could be replaced with a(n ordered) set of index values, in which case X_{3} must be an ordered list of replacement values such that they have the same cardinality; in this situation, the nth index in the index set (according to its ordering) corresponds to an entry in X_{1} which gets replaced (respectively) by the nth entry in X_{3}, where the entries in X_{3} are themselves counted/indexed/ordered in the usual manner and with the first/leading/header entry being 1st unless somehow explicitly specified otherwise; the index set is automatically ordered according to the same ordering as on X_{3} unless explicitly specified otherwise. Example: bai'i'i((A, B, C, D, E, F, G), Set(5, 2), (b, e), 1) = (A, b, C, D, e, F, G), where the ordering on X_{2} and X_{3} is the standard ordering starting with 1; note that Set(5, 2) is unordered and that the standard ordering rearranges it as (2, 5) with 2 being the 1st entry/element, meaning that the index 2 (corresponding with B in X_{1}) gets linked with the first entry of X_{3}, which is b, meaning that B gets replaced by b in X_{1}.

- fau'i
*(exp!)* - mekso ternary operator: inverse function of input function X
_{1}with respect to its input X_{2}, taken on branch or restricted domain X_{3}("domain" being of X_{1}).