No longer use option for local updates.
Instead, I separate it into a total function and a predicate describe whether the action is allowed or not. This has some advantages: * It is much easier to deal with total functions and predicates in Coq than with functions into option. * Already existing functions do not need to be wrapped. Instead, when using a local update you end up with a sensible side condition as a Coq Prop. * The definition of local updates (and all CMRA instances) no longer depend on option.