The boolean flag emit_MESG is consulted by HOL_MESG when it attempts to
print its argument. This flag is not commonly used, and it may disappear
or change in the future.
The default value of emit_MESG is true.
EXAMPLE
- HOL_MESG "Joy to the world.";
<<HOL message: Joy to the world.>>
- emit_MESG := false;
> val it = () : unit
- HOL_MESG "Peace on Earth.";
> val it = () : unit