crate::mod_interface!
{
  /// Edge interface.
  layer edge;
  /// Factory of nodes.
  layer factory;
  /// Interface of a graph.
  layer graph;
  /// Interface to identify an instance of somthging, for exampel a node.
  layer identity;
  /// Node interface.
  layer node;
  // /// Node in a ref counted cell.
  // layer node_cell;
}