Potential bug in update.formula when updating offsets

Hi all,

update.formula does not seem to correctly update (i.e. remove in my case)
offset terms.

Here is an example:

update(~x + offset(z), ~ . - offset(z))

~x + offset(z)

Also:
update(~x, ~ . - offset(z))

~x + offset(z)

In both cases, I would expect the result

~x

as  - <term>  should remove <term> from the formula as happens for instance
in:

update(~x + z, ~ . - z)

~x

I don't know if this behavior is intentional but I would say it is at least
unfortunate.

Paul