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More straightforward

sage: sum = 15
sage: xmin = 1
sage: xmax = 9
sage: ymin = 1
sage: ymax = 9
sage: for x in range(max(xmin, sum-ymax), min(xmax, sum-ymin)+1):
....:     print(x, sum-x)     
6 9
7 8
8 7
9 6