Predicate combinations / RO chains

[cbizon]

Currently there are two thoughts for using edges in coalescence.

1. only coalesce when edge preds are the same
2. Ignore edge preds

But different edges might end up doing the same thing overall. Something like

A-[upregulates]->B and A-[downregulates]->C

Suppose B->[downregulates]->D and C-[upregulates]->D. Then D should be brought in as an extra node, merging these two, since the net effect of them is the same.