Selecting mumamo modes

[Richard M. Stallman]

Here's an idea.  Suppose that `normal-mode' sees that a file specifies
a major mode of the form `A+B'.  If there is no function `A+B-mode',
it could create one using mumamo, based on `A-mode' and `B-mode', and
then call it.

That is safe because it only lets a file call `A-mode' and `B-mode',
which files can already do individually.

Naturally the feature should handle cases of more than two modes in a
similar way.