Bb2 optimization

Optimization of the BB2 scheme.
I suspect you're aware, but if `A(x) = B(x) - C(x)`, then `max(B(x)) - min(C(x))` may not be a tight bound on `A(x)`, since `max(B(x))` and `min(C(x))` are probably at different values of `x`.  It does at least provide a valid upper bound though.