A Torsor[V, R] requires an AbGroup[R] and provides Action[V, R], plus a diff operator, <-> in additive notation,
such that:
1. `(g <-> g) === scalar.id` for all `g` in `G`.
2. `(g <-> h) +> h === g` for all `g`, `h` in `G`.
3. `(g <-> h) +> i === (i <-> h) +> g` for all `g`, `h` in `G`.
4. `(g <-> h) === -(h <-> g)` for all `g`, `h` in `G`.
- Companion
- object