`PartialOrd` for `Variable` violates `transitivity` and `duality` properties

[BoyuanFeng]

Summary of Bug

PartialOrd for Variable violates transitivity and duality properties.

This code prints both 0<1 and 1<0.

See playground for example:

one is Some(Less) than zero
zero is Some(Less) than one

Version

v0.3.0
8bb375e

Steps to Reproduce

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=51b860bb4a8854d6636e740e8be1c9ce