Wrong proof tree returned on NEV query

[mkroetzsch]

The NEV issue #8 appears to be a Nemo problem. Here is the original description on how to reproduce it in NEV.

Steps to reproduce:

• Run the basic ancestor example
• Open proof tree for fact "ancestor(alice,bob)" (the tree is linear, with three nodes)
• Select "unrestrict query" on the right
• Open the root node (it shows two facts: alice,bob and cho,daniel)
• Click the magnifier "Query for this fact" next to cho,daniel

Expected result: The proof tree for ancestor(cho,daniel) is shown.

Actual result: The proof tree for ancestor(alice,daniel) is shown. This tree also has a completely different structure (not linear).

As @menocsk27 wrote in the other issue thread, Nemo seems to return the wrong answer to the right query here.

[aannleax]

Addressed in #750

[monsterkrampe]

It seems like this is still broken, maybe in a different way.

1. Use the same example.
2. Trace ancestor(alice, daniel).

Expected: Get Proof Tree for ancestor(alice, daniel).
What actually happens: Proof Tree for ancestor(cho, daniel) is shown.

I tested this locally with the latest commits on main respectively.