Completed the untested implementation of deferred factories - and there is a bug revealed in an example test. Not sure if this has been in for a while, so I'll commit and then go back and find the first point of failure.
Fix bug observed in example test for generating graphs - 'CollectAcrossMaps' must handle the trivial case of an empty list of maps.
Use sorted association lists rather than maps to map from deferral budget to node tree.