[Python-ideas] Fast sum() for non-numbers

[Stephen J. Turnbull]

Andrew Barnert writes:

 > Meanwhile sum is the obvious way to sum things that are obviously
 > summable (numbers, matrices, etc.), and nothing else.

My intuition matches yours, but I find this argument (and the rest of
the arguments that say that "generic sum() is unobvious and wrong")
logically unsatisfactory.  It would be nice if you could provide a
plausible definition of "summable" other than "__add__() is
implemented".  I don't have one. :-(