Use get_greatest_modulus more consistently, instead of doing the
same thing in an ad-hoc manner in this one place.

Ashutosh Bapat

Discussion: http://postgr.es/m/CAFjFpReT9L4RCiJBKOyWC2=i02kv9uG2fx=4Fv7kFY2t0SPCgw@mail.gmail.com