added Graphviz visualizations

src/runtime/c/pgf/pgf.h

@@ -878,6 +878,12 @@ typedef struct {
     const char *leafEdgeStyle;
 } PgfGraphvizOptions;
 
+PGF_API_DECL PgfText *
+pgf_graphviz_abstract_tree(PgfDB *db, PgfRevision revision,
+                           PgfExpr expr, PgfMarshaller *m,
+                           PgfGraphvizOptions* opts,
+                           PgfExn *err);
+
 PGF_API_DECL PgfText *
 pgf_graphviz_parse_tree(PgfDB *db, PgfConcrRevision revision,
                         PgfExpr expr, PgfPrintContext *ctxt,