Walter Banks wrote:

> The only error case left is the double transition
> (00 -> 11, 01 -> 10, 10 -> 01, 11 -> 00)

I've thought about this.
What I've done is to have analyse steps into
sign (direction) and magnitude.

If I do see a double step, I assume the direction
is the same as before. Inertia (considerable on
a 1-tonne 'scope) suggests this is reasonable.

Double-steps tend to appear when moving fast,
so the inertial assumption becomes even more valid.