Modify new traversal types so that they don't error out when a vertex filter is used (not all parents will be observed at time of vertex index assignment -- but all the parents that matter will)
Cosmetics
Add duplicate() method for testing purposes
More cosmetics
Allow base level UnpackedDagWalker to actually evaluate higher layers' vertex filters so that vertex indexing for depth-first and breadth-traversal don't end up taking exponential time.
Fix unruly error message
Restore depth-first as the default traversal type