Assumed Implicit ordering in Cache is not guaranteed

register_variables!(cache, dynamic_variablenames=(:a,:b),
                           rhs_variablenames=(:rhs_a,:rhs_b))

Because VariablesDict currently uses an unsorted Dict under the hood, wrapping arrays with wrap_dynamic_variables!(cache, u), wrap_rhs_variables!(cache, u) does not guarantee that the vectors are wrapped as u = [<a>, <b>], du = [<rhs_a>, <rhs_b>].

I think that is what happened in !6 (merged) and lead to the problem of LLF not working (which is just a coincidence, since the real problem lies in the variables q,E not evolving).

TODO: Move to ordered dicts in VariablesDict to remove the ambiguity when wrapping.