Ok, let's close this.

I'm fine marking this as closed. While set<...> is rarely the best solution, we
have much more pressing things to worry about in the search code. If you agree,
please close this.

This issue is already quite old and it is not clear what should be done here. Malte?

I see. I faintly remember discussing this in the past, but I guess I forgot the
details.

Sets are not great performance-wise either, but let's leave this as is for now
to avoid messing up the evaluation order shortly before the IPC. I'd suggest
revisiting this later though, so I'll leave this one open.

preferred_ops is a set - exactly because of that.

set<const Operator *> preferred_ops;

Classifying this as "bug" since I see it as a performance bug.

This line in the eager search code:
       bool is_preferred = (preferred_ops.find(op) != preferred_ops.end());
is no good. There are cases where we cannot afford a linear search in this case.

One temporary fix would by introducing additional "markers" in the operators
like the one that exist already.