### Graphs
[ Type directed-graphs Funcon is-cyclic Funcon topological-sort ] Meta-variables GT <: ground-values Type directed-graphs(GT) ~> maps(GT, sets(GT))
*/ Built-in Funcon is-cyclic(_:directed-graphs(GT)) : =>booleans Built-in Funcon topological-sort(_:directed-graphs(GT)) : =>(GT)*
/* `topological-sort(DG)` returns a topological ordering of the vertices
of the graph `DG`, as a sequence of vertices, provided that `DG` is not
cyclic.
*/
/*`directed-graphs(GT)` models directed graphs with vertices of type `GT`, represented as maps from vertices to the set of vertices to which the vertex has an edge. E.g., the graph (1)--->(2) would be represented as `{ 1 |-> {2}, 2 |-> {} }`