code: int createUserIndex() {
  		if (m_indices == null)
  			m_indices = new ArrayList<AttributeStreamOfInt32>(0);
  
  		// Try getting existing index. Use linear search. We do not expect many
  		// indices to be created.
  		for (int i = 0; i < m_indices.size(); i++) {
  			if (m_indices.get(i) == null) {
  				m_indices.set(i, (AttributeStreamOfInt32) AttributeStreamBase
  						.createIndexStream(0, -1));
  				return i;
  			}
  		}
  
  		m_indices.add((AttributeStreamOfInt32) AttributeStreamBase
  				.createIndexStream(0, -1));
  		return m_indices.size() - 1;
  	}
docs: store an integer user value on the vertex.