sumB <- sumB + i * hist.data.[i]
let mB = (float sumB) / (float wB)
let mF = (sum - float sumB) / (float wF)
let between = (float wB) * (float wF) * (mB - mF) ** 2.;
sumB <- sumB + i * hist.data.[i]
let mB = (float sumB) / (float wB)
let mF = (sum - float sumB) / (float wF)
let between = (float wB) * (float wF) * (mB - mF) ** 2.;