Currently, during symbol simplification we remove the original member symbol
from the equivalence class (`ClassMembers` trait). However, we keep the
reverse link (`ClassMap` trait), in order to be able the query the
related constraints even for the old member. This asymmetry can lead to
a problem when we merge equivalence classes:
ClassA: [a, b] // ClassMembers trait,
a->a, b->a // ClassMap trait, a is the representative symbol
Now let,s delete `a`:
Let's merge ClassA into the trivial class `c`:
ClassA: [c, b]
c->c, b->c, a->a
Now, after the merge operation, `c` and `a` are actually in different
equivalence classes, which is inconsistent.
One solution to this problem is to simply avoid removing the original
member and this is what this patch does.
Other options I have considered:
1) Always merge the trivial class into the non-trivial class. This might
work most of the time, however, will fail if we have to merge two
non-trivial classes (in that case we no longer can track equivalences
2) In `removeMember`, update the reverse link as well. This would cease
the inconsistency, but we'd loose precision since we could not query
the constraints for the removed member.