Refinement and Transitivity: What makes GRAIL different

GRAIL differs from most other description logics and frame systems in providing extra facilities to cope with transitive relations and the interaction of transitive relations and subsumption. These are amongst the most powerful features of GRAIL. They are what allows GRAIL to deal cleanly with part-whole and causal relations. However, they can also be confusing, particularly the operator for co-ordinating transitive relations and subsumption which is known as refinedAlong or specialisedBy. (The fact that there are two different variants of the syntax reflects the difficulty we have had explaining it to others.)