prop.gd: tweak TrueMethods

Make it more explicit (just to be safe) that these methods should
only apply to digraphs.

Also add a couple more.

gap/prop.gd

@@ -73,12 +73,14 @@ DeclareSynonymAttr("IsQuasiorderDigraph", IsPreorderDigraph);
 DeclareOperation("DIGRAPHS_IsMeetJoinSemilatticeDigraph",
                  [IsHomogeneousList]);
 
+InstallTrueMethod(IsAcyclicDigraph, IsChainDigraph);
+InstallTrueMethod(IsAcyclicDigraph, IsDigraph and IsDirectedTree);
 InstallTrueMethod(IsAcyclicDigraph, IsEmptyDigraph);
 InstallTrueMethod(IsAcyclicDigraph, IsTournament and IsTransitiveDigraph);
 InstallTrueMethod(IsAntisymmetricDigraph, IsAcyclicDigraph);
-InstallTrueMethod(IsAntisymmetricDigraph, IsTournament);
+InstallTrueMethod(IsAntisymmetricDigraph, IsDigraph and IsTournament);
 InstallTrueMethod(IsBipartiteDigraph, IsCompleteBipartiteDigraph);
-InstallTrueMethod(IsBipartiteDigraph, IsUndirectedForest);
+InstallTrueMethod(IsBipartiteDigraph, IsDigraph and IsUndirectedForest);
 InstallTrueMethod(IsCompleteMultipartiteDigraph, IsCompleteBipartiteDigraph);
 InstallTrueMethod(IsConnectedDigraph, IsBiconnectedDigraph);
 InstallTrueMethod(IsConnectedDigraph, IsStronglyConnectedDigraph);
@@ -88,19 +90,22 @@ InstallTrueMethod(IsHamiltonianDigraph,
 InstallTrueMethod(IsInRegularDigraph, IsRegularDigraph);
 InstallTrueMethod(IsOutRegularDigraph, IsRegularDigraph);
 InstallTrueMethod(IsPreorderDigraph, IsPartialOrderDigraph);
+InstallTrueMethod(IsReflexiveDigraph,
+                  IsDigraph and IsVertexTransitive and DigraphHasLoops);
 InstallTrueMethod(IsRegularDigraph, IsInRegularDigraph and IsOutRegularDigraph);
-InstallTrueMethod(IsRegularDigraph, IsVertexTransitive);
+InstallTrueMethod(IsRegularDigraph, IsDigraph and IsVertexTransitive);
 InstallTrueMethod(IsStronglyConnectedDigraph,
                   IsConnectedDigraph and IsSymmetricDigraph);
 InstallTrueMethod(IsStronglyConnectedDigraph, IsCycleDigraph);
 InstallTrueMethod(IsStronglyConnectedDigraph, IsEulerianDigraph);
 InstallTrueMethod(IsStronglyConnectedDigraph, IsHamiltonianDigraph);
-InstallTrueMethod(IsStronglyConnectedDigraph, IsUndirectedTree);
+InstallTrueMethod(IsStronglyConnectedDigraph, IsDigraph and IsUndirectedTree);
 InstallTrueMethod(IsSymmetricDigraph, IsCompleteDigraph);
-InstallTrueMethod(IsSymmetricDigraph, IsUndirectedForest);
+InstallTrueMethod(IsSymmetricDigraph, IsDigraph and IsUndirectedForest);
 InstallTrueMethod(IsTransitiveDigraph, IsTournament and IsAcyclicDigraph);
-InstallTrueMethod(IsUndirectedForest, IsUndirectedTree);
+InstallTrueMethod(IsUndirectedForest, IsDigraph and IsUndirectedTree);
 
 InstallTrueMethod(DigraphHasEdges, IsDigraph and DigraphHasLoops);
 InstallTrueMethod(DigraphHasLoops, IsReflexiveDigraph and DigraphHasVertices);
 InstallTrueMethod(DigraphHasVertices, IsDigraph and DigraphHasEdges);
+InstallTrueMethod(DigraphHasVertices, IsDigraph and IsDirectedTree);

gap/prop.gi

@@ -334,10 +334,6 @@ end);
 InstallMethod(IsReflexiveDigraph, "for a digraph", [IsDigraph],
 D -> ForAll(DigraphVertices(D), x -> IsDigraphEdge(D, x, x)));
 
-InstallImmediateMethod(DigraphHasLoops, "for a reflexive digraph",
-IsReflexiveDigraph, 0,
-D -> DigraphNrVertices(D) > 0);
-
 InstallMethod(DigraphHasLoops, "for a digraph with adjacency matrix",
 [IsDigraph and HasAdjacencyMatrix],
 function(D)