Add DomainForAction mock for actions on digraphs

gap/oper.gi

@@ -1087,6 +1087,16 @@ function(D, perms)
   return OnDigraphs(D, perms[1]);
 end);
 
+# DomainForAction returns `true` instead of a proper domain here as a stopgap
+# measure to ensure that the Orbit method of GAP uses a SparseHashTable instead
+# of a DictionaryByPosition.
+# As SparseIntKey for digraphs does not depend on the domain, and always
+# returns DigraphHash, the actual returned value of DomainForAction does not
+# matter as long as it returns something other than `fail`.
+InstallMethod(DomainForAction, "for a digraph, list or collection and function",
+[IsDigraph, IsListOrCollection, IsFunction],
+ReturnTrue);
+
 #############################################################################
 # 5. Substructures and quotients
 #############################################################################

tst/standard/oper.tst

@@ -211,6 +211,10 @@ gap> OnMultiDigraphs(gr1, [(1, 3), (1, 7)]);
 Error, the 2nd entry of the 2nd argument <perms> must permute the edges of the\
  digraph <D> that is the 1st argument,
 
+#  DomainForAction
+gap> DomainForAction(RandomDigraph(10), SymmetricGroup(10), OnDigraphs);
+true
+
 #  InNeighboursOfVertex and InDegreeOfVertex
 gap> gr := DigraphFromDiSparse6String(".IgBGLQ?Apkc");
 <immutable multidigraph with 10 vertices, 6 edges>