__PS_MV_REG is used to return multiple values from functions. In Parenscript version 2.6, it was set only when you actually RETURNED multiple values. In 2.7, it is set to
"use strict";, this will cause a runtime error, unless you include the following before that code executes:
This declaration either needs to be added somewhere or, since I can't think of a good place to put it, other than the header of the HTML file including the Parenscript output, which isn't under Parenscript's control, documented.