write down proof that the error bounding code in fmprb_mul does not overflow
write down proof that the error bounding code in fmprb_mul does not overflow

argriffing/arb,argriffing/arb,fredrik-johansson/arb,fredrik-johansson/arb,argriffing/arb,pascalmolin/arb,pascalmolin/arb,pascalmolin/arb