mekso quaternary operator – Pochhammer symbol: with/for input (X1, X, this word/function outputs \prodk = 0^X2 - 1 (X; by default, X4 = 1 unless explicitly defined otherwise.
For the basic definition, all inputs (especially those which are not X1) should be nonnegative integers; X3 can be further restricted to 0 (for the falling factorial) and 1 (for the rising factorial); X4 > 0 will be typical. X2 < 1 yields the empty product, which is typically defined by convention to be 1, regardless of all other inputs (so long as they are valid/belong to the domain). X2 is the number of terms in the aforementioned defining product and interacts with X4 in somewhat-complicated ways; be careful to avoid multiplying by nonpositive numbers unless such is actually desired (which may break certain recursive formulas); in order to avoid negative terms, enforce that X2 < 1 + min(Set(1, X1 \% X, where: "\%" denotes the modulus/remainder (see: "vei'u") of its left-hand/first input (here: X1) wrt/when integer-dividing it by its right-hand/second input (here: X4); recall: x \% y in [0, y) for all real numbers x and y: y > 0. Also, X4 = 1 by default. See also: "ne'o", "ne'oi".