RE: bind commands that change buffer contents to `undefined'whenread-only?

[Drew Adams]

Scrap the implementation I sent. Sorry about that. The general idea is OK,
but it must not override any existing local binding of the key.

It's not enough to check that the command is not bound in the local map; the
key also must not be bound in the local map. So, for instance, even though
`self-insert-command' is not bound in `dired-mode-map', that's not enough to
determine that we should substitute `undefined' for the bindings of all keys
that are globally bound to `self-insert-command'. `R', for instance is bound
in `dired-mode-map', so it must not be rebound to `undefined'.