digraph: fix String method for chains/cycles

gap/digraph.gi

@@ -728,9 +728,15 @@ function(D)
   Add(strings, String(outnbs_rep));
 
   N              := DigraphNrVertices(D);
-  props          := [IsCycleDigraph, IsCompleteDigraph, IsChainDigraph,
+  props          := [x -> IsCycleDigraph(x)
+                          and x = CycleDigraph(DigraphNrVertices(x)),
+                     IsCompleteDigraph,
+                     x -> IsChainDigraph(x)
+                          and x = ChainDigraph(DigraphNrVertices(x)),
                      IsEmptyDigraph];
-  creators_props := ["CycleDigraph", "CompleteDigraph", "ChainDigraph",
+  creators_props := ["CycleDigraph",
+                     "CompleteDigraph",
+                     "ChainDigraph",
                      "EmptyDigraph"];
   for i in [1 .. Length(props)] do
     if props[i](D) then

tst/testinstall.tst

@@ -385,6 +385,33 @@ gap> D := Digraph([[2, 2, 2, 2, 2], []]);;
 gap> GeneratorsOfEndomorphismMonoid(D);
 Error, expected a digraph without multiple edges!
 
+# Issue 517: bug in String for digraphs satisfying IsChainDigraph or
+# IsCycleDigraph but not being equal to ChainDigraph or CycleDigraph.
+gap> D := ChainDigraph(4);
+<immutable chain digraph with 4 vertices>
+gap> D := DigraphReverse(D);
+<immutable digraph with 4 vertices, 3 edges>
+gap> IsChainDigraph(D);
+true
+gap> D = ChainDigraph(4);
+false
+gap> String(D);
+"DigraphFromDigraph6String(\"&CACG\")"
+gap> String(ChainDigraph(4));
+"ChainDigraph(4)"
+gap> D := CycleDigraph(4);
+<immutable cycle digraph with 4 vertices>
+gap> D := DigraphReverse(D);
+<immutable digraph with 4 vertices, 4 edges>
+gap> IsCycleDigraph(D);
+true
+gap> D = CycleDigraph(4);
+false
+gap> String(D);
+"DigraphFromDigraph6String(\"&CECG\")"
+gap> String(CycleDigraph(4));
+"CycleDigraph(4)"
+
 #  DIGRAPHS_UnbindVariables
 gap> Unbind(gr2);
 gap> Unbind(gr);