O(n) patches and side effects

[Ralf Wildenhues]

Hi Eric,

I'm looking through a bunch of the O(n) patches, and asking myself:
The fact that rescanning is avoided (by one means for m4-1.4.x and
another in m4-1.6.x), does that add a slight backward incompatibility
if the macro arguments contains side-effects?  I'm thinking of stuff
like m4_syscmd and esyscmd that could affect external state; aren't
they possibly run a different number of times now?  If yes, can we
also think of internal side effects that may matter to users, like
redefinitions or so?

If those issues are present, we should estimate their severity
(I expect them to be rare, and dubious constructs), and write a
note in NEWS (and maybe the manual) about this issue, so that
users know what to expect.

Thanks,
Ralf