From 5c9737d7578d48b40aabf5a4f08c7a3de607a92d Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Wed, 3 Jan 2018 04:02:10 +0100
Subject: [PATCH] Fix forming trivial quotients of algebra modules

---
 lib/algrep.gi              |  9 +++++++++
 tst/testinstall/algrep.tst | 15 +++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/lib/algrep.gi b/lib/algrep.gi
index 3ef1c1f2a3b..6a197df6152 100644
--- a/lib/algrep.gi
+++ b/lib/algrep.gi
@@ -1194,6 +1194,15 @@ InstallMethod( NaturalHomomorphismBySubAlgebraModule,
     f:= NaturalHomomorphismBySubspace( V, W );
     quot:= ImagesSource( f );
     
+    if IsTrivial(quot) then
+        quot := SubAlgebraModule( V, [], "basis" );
+        nathom := ZeroMapping( V, quot );
+        SetIsSurjective( nathom, true );
+        SetImagesSource(nathom, quot);
+        SetKernelOfAdditiveGeneralMapping( nathom, W );
+        return nathom;
+    fi;
+
     imgs:= List( Basis( V ), x -> Coefficients( Basis( quot ),
                    ImagesRepresentative(f,x)));
 
diff --git a/tst/testinstall/algrep.tst b/tst/testinstall/algrep.tst
index 3535237179f..4ebe420b988 100644
--- a/tst/testinstall/algrep.tst
+++ b/tst/testinstall/algrep.tst
@@ -55,6 +55,21 @@ gap> S1:=SubAlgebraModule(S,bas,"basis");
 <16-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
 
 #
+# factors
+#
+gap> S0/S0;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S1/S0;
+<16-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S1/S1;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S0;
+<35-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S1;
+<19-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+gap> S/S;
+<0-dimensional left-module over <Lie algebra of dimension 4 over GF(3)>>
+
 #
 # cocycles etc.
 #