X1 must be some ordered and indexed structure; X2 must be an index which specifies an entry/element/term which actually appears in X1; X3 is an alternative value of appropriate type which can, and after this operation does, replace the X2th 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 X4 specifies otherwise (in which case, it is according to that specification). Exactly the X2th entry is reppaced, and it is replaced with exactly X3. For example: in (A, B, C), A is the X4th entry (default: 1st); bai'i'i((A, B, C), 2, b, 1) = (A, b, C). X2 itself could be replaced with a(n ordered) set of index values, in which case X3 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 X1 which gets replaced (respectively) by the nth entry in X3, where the entries in X3 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 X3 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 X2 and X3 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 X1) gets linked with the first entry of X3, which is b, meaning that B gets replaced by b in X1.