code: @SuppressWarnings("unchecked")
  	@Override
  	public PairSet<T, I> convert(Collection<?> c) {
  		if (c == null)
  			return empty();
  
  		// useless to convert...
  		if (hasSameIndices(c))
  			return (PairSet<T, I>) c;
  
  		// convert
  		PairSet<T, I> res = empty();
  		for (Pair<T, I> p : (Collection<Pair<T,I>>) c)
  			res.matrix.add(transactionToIndex(p.transaction), itemToIndex(p.item));
  		return res;
  	}
docs: {@inheritDoc}