code: public void subtract(R1 relationships) {
          if (null == base) {
              throw new AssertionError("");
          }
          for (int i = 0; i < base.length; i++) {
              if (null == base[i]) {
                  continue;
              }
              final IConceptSet set = data[i] = new SparseConceptHashSet();
  
              set.addAll(base[i]);
              if (null != relationships.data[i]) {
                  set.removeAll(relationships.data[i]);
              }
          }
      }
docs: This should only ever be called when the relationships wrap an initial
  state and no other methods have been called.
  
  @param relationships