From 61be0c98ebf01759839b01ec22e36a351b51d3d6 Mon Sep 17 00:00:00 2001
From: "James D. Mitchell" <jdm3@st-andrews.ac.uk>
Date: Wed, 27 Mar 2024 15:10:37 +0000
Subject: [PATCH] Add IsModularLatticeDigraph

---
 gap/prop.gd |  1 +
 gap/prop.gi | 21 +++++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/gap/prop.gd b/gap/prop.gd
index e58a20579..553c683fb 100644
--- a/gap/prop.gd
+++ b/gap/prop.gd
@@ -51,6 +51,7 @@ DeclareProperty("IsJoinSemilatticeDigraph", IsDigraph);
 DeclareProperty("IsMeetSemilatticeDigraph", IsDigraph);
 DeclareProperty("IsPermutationDigraph", IsDigraph);
 DeclareProperty("IsDistributiveLatticeDigraph", IsDigraph);
+DeclareProperty("IsModularLatticeDigraph", IsDigraph);
 DeclareSynonymAttr("IsLatticeDigraph",
                    IsMeetSemilatticeDigraph and IsJoinSemilatticeDigraph);
 DeclareSynonymAttr("IsPreorderDigraph",
diff --git a/gap/prop.gi b/gap/prop.gi
index 0f4fd0d18..33b0b2660 100644
--- a/gap/prop.gi
+++ b/gap/prop.gi
@@ -661,19 +661,28 @@ end);
 
 InstallMethod(IsDistributiveLatticeDigraph, "for a digraph", [IsDigraph],
 function(D)
-  local N5, M3;
+  local M3;
+
   if not IsLatticeDigraph(D) then
     return false;
   fi;
 
-  N5 := DigraphReflexiveTransitiveClosure(
-        Digraph([[2, 4], [3], [5], [5], []]));
   M3 := DigraphReflexiveTransitiveClosure(
         Digraph([[2, 3, 4], [5], [5], [5], []]));
 
-  if LatticeDigraphEmbedding(N5, D) <> fail or
-       LatticeDigraphEmbedding(M3, D) <> fail then
+  return IsModularLatticeDigraph(D) and
+       LatticeDigraphEmbedding(M3, D) = fail;
+end);
+
+InstallMethod(IsModularLatticeDigraph, "for a digraph", [IsDigraph],
+function(D)
+  local N5;
+  if not IsLatticeDigraph(D) then
     return false;
   fi;
-  return true;
+
+  N5 := DigraphReflexiveTransitiveClosure(
+        Digraph([[2, 4], [3], [5], [5], []]));
+
+  return LatticeDigraphEmbedding(N5, D) = fail;
 end);