code: protected IConceptSet getA(int B, int r) {
          if (B >= CONCEPTS) {
              resizeConcepts(B);
          }
          if (r >= ROLES) {
              resizeRoles(r);
          }
          // Note the "+1" in the following line:
          final int index = indexOf(B, r) + 1;
          if (null == data[index]) {
              data[index] = new SparseConceptSet();
              addRole(r);
  
              if (null != base && index < base.length && null != base[index]) {
                  data[index].addAll(base[index]);
              }
          }
          return data[index];
      }
docs: Returns {A | A [ r.B} or {A | (A,B) in R(r)}
  
  Lazily initialise data from base as a side-effect of this call.
  
  @param B
  @param r
  @return