What is whichMenu?

[Dr. Robert E. Ball]

When using the browser pointer, I go to the menubar of my OS X stack, click
on a menu item, e.g. File, and then click on one of the dropdown items in
the menu, e.g. Save in the File menu. In the message watcher, I get

MouseDown  -- I clicked on File
cREVGetsUpdate
MenuPick   -- I clicked on Save
whichMenu

when it works properly. However, on another stack it doesn't work. I only
get

mouseDown
cREVGetsUpdate
menuPick


No whichMenu. ???  I don't know where to look to find out where the problem
is located. I searched the PDF manual and Googoled whichMenu. No luck. Does
anyone know where whichMenu is described?