code: public void composeLeftLeftInv(IntBitRelation left, IntBitRelation right) {
          int i, a, b;
          IntBitSet li, ri;
  
          for (i = 0; i < line.length; i++) {
              li = left.line[i];
              ri = right.line[i];
              if (li != null && ri != null) {
                  for (a = li.first(); a != -1; a = li.next(a)) {
                      for (b = ri.first(); b != -1; b = ri.next(b)) {
                          add(b, a);
                      }
                  }
              }
          }
      }
docs: If (x,a) is element of left and (x,b) is element of right,
  (b,a) is added to this relation.
  @param left  left relation
  @param right right relation