Signals should propagate upward through the tree of nodes:
  either explicitly as called within procedures
  or implicitly as the nodes are organized

What should we do when we run out of memory space?
  Bomb for now, and improve upon so over time.
  Note improvements as they are made.

Undo:
  Audit trail  Try for infinite depth
  Give UNDO a good user interface, add a history and selection mechanism.