http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/mirrors/gmpdoc/Small-Quotient-Division.html
Instead a division of just the top 2*Q limbs of the dividend by the top Q limbs of the divisor can be
used to form a trial quotient. This requires only those limbs normalized, not
the whole of the divisor and dividend. A multiply and subtract then applies the trial quotient to the M−Q
unused limbs of the divisor and N−Q dividend limbs (which includes Q
limbs remaining from the trial quotient division).