When 'integrate' returns a list of results, there are conditions
associated with them. More specifically, it tries to ensure
(y)^(1/n) is real when n is even, aka ensure y is positive.
So in the example of 'integrate(1/(x^4+2*a*x^2+a^2-1),x)',
the first answer, (-a-1) and (-a+1) should be positive, aka a <=-1
The second, (-a-1) and (a-1) should be positive, which is impossible.
(Current code checks sign of the product.)
The third (a+1) and (-a+1) should be positive, aka -1<=a<=1
The fourth (a-1) and (a+1) should be positive, aka a>=1.
Clearly there are much room for improvement in the algorithm.
See 'irexpand.spad', especially the function 'pairsum' and
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