Date: Sat, 16 May 1998 19:20:24 -0400

From: Alex Bogomolny

The questions you ask.

I am pretty sure that in different circumstances both possibilities may prove useful. However, consider this:

sum(A union B) = sum(A) + sum(B)

may be expected to be correct for A and B with empty intersection. In particular, this would imply sum(empty) = 0.

But then again, if you want to get a warning of an attempt to sum over an empty set then the function sum() may as well return NaN. Up to you.

Best regards,

Alexander Bogomolny