Description: The square root function must make a choice between the two roots, which
differ by a sign change. In the general complex case, the choice of
"positive" and "negative" is not so clear. The convention we use is to
take the root with positive real part, unless A is a nonpositive real
(in which case both roots have 0 real part); in this case we take the one
in the positive imaginary direction. Another way to look at this is that
we choose the root that is largest with respect to lexicographic order on
the complex numbers (sorting by real part first, then by imaginary part as
tie-breaker). (Contributed by Mario Carneiro, 10-Jul-2013)