I'm about to want to expand the underlying number-theory code, so I'll
start by moving it into a file where it has room to grow without
swamping the main purpose of eccref.py.
The __truediv__ pair makes the whole program work in Python 3 as well
as 2 (it was _so_ nearly there already!), and __int__ lets you easily
turn a ModP back into an ordinary Python integer representing its
least positive residue.
