I very much doubt that Num a is sufficient. That's not even enough to check whether a number is even. You can certainly perform the calculation with `Integral a`, but you'll have to apply some external reasoning to see that the result is correct.
Some nominally rational functions, e.g n*(n+1)/2,
yield integer values for integer arguments. I seek
either a way to wrap such a function so it has type
Num a => a->a or a convincing argument that it can't
be done.
Doug
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