Handle symmetric relationships

[cbizon]

he first thing that we do is look for answers that can be smushed together. We currently have a strict definition. Two answers must be exactly the same, except for the identity of one node. "exactly the same" extends to edges. So let's say that our question is A-B. We'll take any edge type.

It's very easy to end up with cases where A and B are connected by multiple edge types. Our strictness means that all edge directions must match. But some edges are symmetric. For example 'related_to' or 'correlated_with'

So for instance, A-[related_to]->B will merge with A-[related_to]->B' but not with A<-[related to]-B'.

That's wrong because there's no real reason to favor one direction over another. Note that this needs to happen both for edges attached to our candidate node, but also to the whole knowledge graph.

Probably a simple way to do this is to immediately cycle through the KG, looking for symmetric edge types, and flipping them so that they are all have source_id as the lexigraphically low node in the relationship.

[cbizon]

One aspect of this will involve finding out which relations are symmetric. Probably should do a PR on biolink and then pull this from bl-lookup.