sum(A1:A3) and cut/paste A2 or A3

[Heath Martin <martin math ucf edu>]

Using a fresh compile of Gnumeric 1.1.17:

If A4 is set to sum(A1:A3) and cells A1 to A3 have numeric values,
and then cell A2 is cut and pasted to B2, then cell A4 still
has the old sum.  It is not updated like it should be.

This is nearly identical to Bug #106148, except that that bug dealt only
with the case that endpoints are moved.

Also, if A3 is cut and pasted to B3, and then cut and pasted back
to A3, then a #REF! error is raised in the sum.  Bug #106148 states
that this is expected behavior;  could someone explain why?  If
this is expected, then shouldn't a #REF! error be raised if
instead I cut/paste B2 back to A2?  That is, as it stands now,
a #REF! error is raised only when pasting onto the endpoints
of the range in the sum().  Really though, I would think that
pasting onto any part of the range in the sum() should not
be an error.  After all, I could just manually enter a new value.

I've been trying to understand the code well enough to fix these,
but the relevant logic in expr.c is pretty dense.

Best Regards,
Heath Martin