RE: Leaving out non-applicable commands on Mx

[Drew Adams]

> I sense ways in which this could go wrong: expecting to find
> a command, failing for some undiscovered reason X, and then
> the user believing no such command exists.

Yes.  And from the outset the user will have only a restricted
view of what commands are currently available, because an Emacs
developer will have made a priori assumptions about which commands
are currently appropriate.

If that's really important for some specific mode then it
is easy enough to implement that for a given mode.  It should
not be imposed on all of Emacs (e.g., via `M-x').

The specific command reading input should filter as is
appropriate for it, using a predicate - as has always been
the case.  And (IMHO), no such filter is appropriate for
`M-x', which should remain general.

Of course, nothing prevents someone from defining a command
similar to `M-x' but that is more restrictive in the sense
suggested (simple to do).  Just please don't bind it by
default to `M-x'.

> Core Emacs behavior doesn't need to change to demonstrate this
> feature, does it? Write a new execute-extended-command and see
> if it works as well as you hope.

Precisely (only please don't call it `execute-extended-command' ;-)).