properGeneralizedInverseGaussianVariance

import std.math : approxEqual;
auto expectation     = properGeneralizedInverseGaussianMean!double(2.0, 3.0, 4.0);
auto secondRowMoment = properGeneralizedInverseGaussianMean!double(2.0, 3.0, 4.0, 2);
auto variance        = properGeneralizedInverseGaussianVariance!double(2.0, 3.0, 4.0);
assert(approxEqual(secondRowMoment - expectation^^2, variance));