code: int createGeometryUserIndex() {
  		if (m_geometry_indices == null)
  			m_geometry_indices = new ArrayList<AttributeStreamOfInt32>(4);
  
  		// Try getting existing index. Use linear search. We do not expect many
  		// indices to be created.
  		for (int i = 0; i < m_geometry_indices.size(); i++) {
  			if (m_geometry_indices.get(i) == null) {
  				m_geometry_indices.set(i,
  						(AttributeStreamOfInt32) AttributeStreamBase
  								.createIndexStream(0));
  				return i;
  			}
  		}
  
  		m_geometry_indices.add((AttributeStreamOfInt32) AttributeStreamBase
  				.createIndexStream(0));
  		return m_geometry_indices.size() - 1;
  	}
docs: stores -1 for that geometry.