Reevaluate the constraints on non-manifold topology in graphs. about plexus HOT 2 OPEN

I think this issue (or others; I'm not yet sure how to organize the work) should track the following changes to graphs:

• Disallow singularities. See 7de93ea, which removed their detection in the mutation API.
• Disallow unbounded edges.
• Require an arity of three or higher for rings. (This is implied by the previous item.)

In essence, this restores the requirement (and limitation) that graphs represent a manifold, with the possible exception of empty rings.

from plexus.

Another important observation that's worth noting here: disallowing these non-manifold features has implications for the way that topological mutations work, especially removals. For example, when removing a face, it may be necessary for other topology to also be removed so as to avoid inconsistency.

One obvious example is a graph composed of nothing more than a single face. Removing the face (and nothing more) results in unbounded edges. In this situation, removing the face would need to be maximally destructive, resulting in the removal of all edges, arcs, and vertices too.

from plexus.