Elliptic curve arithmetic in the Montgomery representation

[Richard Heylen <richardheylen@gmail.com>]

I have some calculations I would like to do with points on an elliptic
curve over a ring without having the benefit of knowing the y
coordinate. I believe the best way of dealing with this problem is
using the Montgomery representation which is probably implemented in
pari at some level due to the presence of ECM factoring routines.

I was wondering if there's any support for these sorts of calculations
or whether someone has a suitable library developed which would save
me writing it all myself.

Cheers!

Richard