Use anscestor relations in predicates

[cbizon]

Currently there are two thoughts for using edges in coalescence.

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

One extension: Use predicate subclasses. So don't require perfect equality, but allow subclassing. If A-[increases expression of]-B and A-[related to]-C, then B and C should be allowed to coalesce with a predicate of the superclass (related to).

In fact, if the edge preds are ignored, maybe we should consider that as merging at the lowest common superproperty.