Every menu has an associated pattern match buffer. As input
events that are printable characters come in,
they are appended to this match buffer and tested for a match,
as described in
set-menu-pattern! sets the pattern buffer for the
given menu and tries to find the first matching item. If it succeeds,
that item becomes current; if not, the current item does not change.
It may return the following error codes:
E_BAD_STATE if the outline was called
from an initialization or termination function,
the character failed to match, or
The procedure menu-pattern returns the pattern buffer of the given menu.