This is part of meta issue987.

Add link to meta issue.

One of the reasons this issue is non-trivial is the existence of the LandmarkGraph::get_landmark(const FactPair &fact) method which requires that every fact is associated with at most one landmark. It's possible to associate each fact with every landmark it occurs in and return a vector of LandmarkNodes in the function instead, but the consequences of such a change are not clear yet.

One of the main reasons we have problems with merging landmark graphs is that 
the code does not support having both A and (A or B) as landmarks, even though A 
or B might have more orderings.

One solution is to get rid of the limitation of having each fact appear in at 
most one landmark.