#86 Precedence - multiple orderings not supported

[Jonathan Kilgour]

It's written into the data model that nodes can be in multiple orderings wrt each other. That's to say $a<>$b and $b<>$a can both hold (but with respect to a different common ancestor).

The current precedence operator only looks for the first common ancestor; any others are simply ignored. This is a bug, though not one that will commonly be discovered.

It will be worth fixing when we're adding in things like horizontal and vertical distance and specifying the type of the common ancestor node.