init commit again

IBAF-cbs/Funcons-beta/Values/Composite/Graphs/Graphs.cbs

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+### Graphs
+
+[
+  Type   directed-graphs
+  Funcon is-cyclic
+  Funcon topological-sort
+]
+
+Meta-variables
+  GT <: ground-values
+
+Type
+  directed-graphs(GT) ~> maps(GT, sets(GT))
+/*
+  `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 |-> {} }`  
+*/
+
+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.
+*/