Dismissing a completion buffer?

[WalterGR]

When a completion buffer pops up, e.g. M-x o, enter something, hit
tab... how do I dismiss that buffer?  Ctrl-g (cancel) seems to have no
effect; Ctrl-x k doesn't do the right thing; and hitting Esc doesn't
have the desired effect.

This:

http://www.dr-qubit.org/predictive/user-guide/html/Pop_002dUp-Frame.html

mentions completion-popup-frame-dismiss and some key bindings.  The
key bindings don't seems to work on my installation.

Thanks,

Walter