code: public IConceptSet lookupConcept(int r) {
          if (r >= data.length) {
              return IConceptSet.EMPTY_SET;
          }
  
          if (null == data[r]) {
              return IConceptSet.EMPTY_SET;
          } else {
              return new ReadonlyConceptSet(data[r]);
          }
      }
docs: Returns the set of concepts associated to the concept in a
  {@link Context} by role r.
  
  @param r
  The role
  @return The set of concepts associated to the concept in the context.