Quoting Malte (https://bitbucket.org/jendrikseipp/downward/pull-requests/58/issue535/diff#comment-24332693):

"I think it's a good idea to avoid all the copying and merging of preferred operators that happens currently 
[...]. Not sure what the best place for the information to live is. Conceptually, it would make sense to me to 
store [preferred operators and similar information] in EvaluationContext and to make that class "resettable" 
(i.e., allow some form of assignment) so that every search engine can use a single reusable evaluation context 
rather than generating new ones all the time. The current code is quite wasteful w.r.t. performance, but we 
decided to live with for now because the old code was so broken and it's a step in the right direction."