And I'm not sure where you'd store the weight,
if not in the restraint. Why not just in the Restraint base class?
The Restraint never actually uses its weight. It is a property of how
the thing calling the restraint combines the weights, not of the
restraint itself. And so should go with the thing calling the restraint.
Plus, this way we can reuse a restraint with different weights in
different places if we should so desire.
My alternative to stacks: restraints which contain other restraints
would use a copy-like constructor to make a cloned
DerivativeAccumulator, multiplied by their own weight, and then pass
that to their child restraints' evaluate methods.