fixed but then eventually removed the first optimization for computing
atomic subsumers because it wasn't any faster or principally different
from the simpler version.
two ways of dealing with property hierarchies: i) generating
propagations when processing fillers for sub-roles of existing links
(and also when the first link for each role is created), ii) generating
propagated subsumers for super-roles of the link being processed. ii)
seems faster atm, not fully sure why.