XCas sum limit confusion

[Albert Chan]

For the same reason, product limits behave exactly the same as sum.

Cas> Pochhammer(a,n) := product((a+j), j, 0, n-1)
Cas> Pochhammer(a, 0)      → 1
Cas> Pochhammer(0, 0)      → 1

When limits get switched, "opposite" for sum negated equivalent to product reciprocal.

Pochhammer(a, -|n|) = product((a+j), j, 0, -|n|-1) = 1/product((a+j), j, -|n|, -1)

Cas> Pochhammer(a, 3)      → a*(a+1)*(a+2)
Cas> Pochhammer(a, -3)     → 1/((a-1)*(a-2)*(a-3))