[O] set global visibility set to CONTENTS in a defun?

[Matt Price]

I am sure it's possible to do this ,but I can't seem to figure out how
to set the global visibility level of org headings to a pre-specified
level in a list function.

(org-global-cycle) cycles the visibility among OVERVIEW --> CONTENTS --> ALL
it accepts an optional argument, but whatever I put in the argument
(number, string, whatever) has the effet of setting visibility to
OVERVIEW, e.g..  I'm sure there's a trick I'm missing, could someone
please tell me what it is?

Thanks,
Matt