IsTwoEdgeTransitive removed support for multidigraphs (#736)

Author: frankiegillis

doc/prop.xml

@@ -1585,8 +1585,8 @@ false]]></Example>
   <Prop Name="IsTwoEdgeTransitive" Arg="digraph"/>
   <Returns><K>true</K> or <K>false</K>.</Returns>
   <Description>
-    If <A>digraph</A> is a digraph, then <C>IsTwoEdgeTransitive</C> returns
-    <K>true</K> if <A>digraph</A> is 2-edge transitive, and <K>false</K>
+    If <A>digraph</A> is a digraph without multiple edges, then <C>IsTwoEdgeTransitive</C>
+    returns <K>true</K> if <A>digraph</A> is 2-edge transitive, and <K>false</K>
     otherwise. A digraph is <E>2-edge transitive</E> if its automorphism group
     acts transitively on its 2-edges via the action
     <Ref Func="OnTuplesTuples" BookName="ref"/>.
@@ -1604,6 +1604,8 @@ gap> IsTwoEdgeTransitive(DigraphByEdges([[1, 2], [2, 3], [3, 1], [3, 4]]))
 false
 gap> IsTwoEdgeTransitive(CycleDigraph(5));
 true
+gap> IsTwoEdgeTransitive(Digraph([[2], [3, 3, 3], []]));
+Error, the argument <D> must be a digraph with no multiple edges,
 ]]></Example>
   </Description>
 </ManSection>

gap/prop.gi

@@ -687,10 +687,17 @@ function(D)
   return LatticeDigraphEmbedding(N5, D) = fail;
 end);
 
-InstallMethod(IsTwoEdgeTransitive, "for a digraph", [IsDigraph],
+InstallMethod(IsTwoEdgeTransitive,
+"for a digraph without multiple edges",
+[IsDigraph],
 function(D)
   local twoEdges;
 
+  if IsMultiDigraph(D) then
+    ErrorNoReturn("the argument <D> must be a digraph with no multiple",
+                  " edges,");
+  fi;
+
   twoEdges := Filtered(Cartesian(DigraphEdges(D), DigraphEdges(D)),
                        pair -> pair[1][2] = pair[2][1]
                                and pair[1][1] <> pair[2][2]);

tst/standard/prop.tst

@@ -1694,6 +1694,8 @@ gap> IsTwoEdgeTransitive(DigraphByEdges([[1, 2]]));
 true
 gap> IsTwoEdgeTransitive(DigraphByEdges([]));
 true
+gap> IsTwoEdgeTransitive(Digraph([[2], [3, 3, 3], []]));
+Error, the argument <D> must be a digraph with no multiple edges,
 
 # DigraphHasNoVertices and DigraphHasAVertex
 gap> List([0 .. 3], i -> DigraphHasAVertex(EmptyDigraph(i)));

tst/testinstall.tst

@@ -436,6 +436,10 @@ rec( distances := [ [ 0, 5 ], [ 5, 0 ] ],
 gap> EdgeWeightedDigraphShortestPath(d, 1, 2);
 [ [ 1, 2 ], [ 1 ] ]
 
+# IsTwoEdgeTransitive
+gap> IsTwoEdgeTransitive(DigraphByEdges([[1, 2], [2, 3], [3, 1]]));
+true
+
 # Issue 617: bug in DigraphRemoveEdge, wasn't removing edge labels
 gap> D := DigraphByEdges(IsMutableDigraph, [[1, 2], [2, 3], [3, 4], [4, 1], [1, 1]]);;
 gap> DigraphEdgeLabels(D);