Separating levels of abstraction is the theme here.
You are given an object graph and are asked to
"invent" a corresponding class graph.


The object graph happens to describe a class graph for class graph.
So you have to jugle three levels of abstraction.

Cd_graph = < first > Adj < rest > Adj_list_ .
Adj = < vertex > Vertex < ns > Neighbors  .
Neighbors : Construct | Alternat .
Construct =  < c_ns > Any_vertex_list_ .
Alternat =  < first > Vertex  < second > Vertex .
Any_vertex_ : Labeled_vertex | Syntax_vertex .
Nany_vertex_list = <first> Any_Vertex_ <rest> Any_vertex_list_.
Any_vertex_list_ : Nany_vertex_list | Empty.
Empty = .
Vertex = Ident.
Labeled_vertex = Ident Vertex.
Syntax_vertex = char.
Adj_list_ : Empty_cd_graph |  Cd_graph.
Empty_cd_graph = .