Semi-contractibility

[bvssvni]

Here, the left middle and right middle nodes are semi-contractible.

It means each have only one child if and only if the other has more than one child.

• If you assign one child to the left middle node, then the right middle node is assigned two children
• If you assign one child to the right middle node, then the left middle node is assigned two children

Adding a check to avatar_connectivity leads to exclusion of Wagner graphs as a filled Avatar Graph. It means rules with Möbius topology are excluded.

I am thinking about this as similar to the distinction between inductive and co-inductive theorem proving. Both create valid theories, but they differ on some edge cases.

[bvssvni]

Decided to allow semi-contractibility, because this property is symmetric.