code: private boolean hasSameIndices(Collection<?> c) {
  		return c != null 
  				&& (c instanceof PairSet) 
  				&& (allTransactions == ((PairSet) c).allTransactions)
  				&& (allItems == ((PairSet) c).allItems);
  	}
docs: Checks if the given collection is a instance of {@link PairSet} with
  the same index mappings
  
  @param c
  collection to check
  @return <code>true</code> if the given collection is a instance of
  {@link PairSet} with the same index mappings