code: public boolean contains(IntBitRelation sub) {
          int i;
  
          if (line.length != sub.line.length) {
              return false;
          }
  
          for (i = 0; i < line.length; i++) {
              if (line[i] == null) {
                  if (sub.line[i] != null) {
                      return false;
                  }
              } else {
                  if ((sub.line[i] != null) &&
                      !line[i].containsAll(sub.line[i])) {
                      return false;
                  }
              }
          }
          return true;
      }
docs: Subset test.
  @param  sub  relation to compare with.
  @return true, if every element of sub is element of this