User blog comment:Hurricane245/Zenna Function/@comment-35470197-20191121143206/@comment-35470197-20191121144451

> When you calculate Z^x(y), repeat Z^x now.

Ill-defined because
 * 1) the number of the repetition is unspecified.
 * 2) it includes the circular logic, i.e. the definition of Z^x refers to itself.