Table of Contents - 21.38.4.35. Additions for square root; absolute value
This is based on the observation that the real and imaginary parts of a
complex number can be calculated from the number's absolute and real part and
the sign of its imaginary part.
Formalization of the formula in sqrtcval was motivated by a short
<A HREF="https://www.youtube.com/watch?v=QAhWTQ4Ravw">Michael Penn video</A>.