2 * hypot -- sqrt(p*p+q*q), but overflows only if the result does.
3 * See Cleve Moler and Donald Morrison,
4 * ``Replacing Square Roots by Pythagorean Sums,''
5 * IBM Journal of Research and Development,
6 * Vol. 27, Number 6, pp. 577-581, Nov. 1983
13 hypot(double p, double q)