PEP: add __sum__ method

[dmitrey]

hi all,
could you consider adding __sum__ method, e.g. Python sum(a) checks
does a have attribute __sum__ and if it has, then a.__sum__() is
invoked instead of Python sum(a).
(for my soft FuncDesigner it would be very essential, I guess for many
other soft as well, e.g. for PuLP, who has to use lpSum, because
ordinary Python sum slows it very much, as well as my funcs, and for
large-scale problems max recursion depth is exeeded).
Regards, D.
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