From feb7cf2c31f6580f13250b3f582d97d1c4b55420 Mon Sep 17 00:00:00 2001
From: Wilf Wilson <wilf@wilf-wilson.net>
Date: Sun, 25 Nov 2018 21:14:15 +0100
Subject: [PATCH] properties: the empty semigroup is not monogenic

---
 gap/attributes/properties.gi | 2 +-
 tst/standard/properties.tst  | 5 +++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/gap/attributes/properties.gi b/gap/attributes/properties.gi
index f4afca5b9..9a108e800 100644
--- a/gap/attributes/properties.gi
+++ b/gap/attributes/properties.gi
@@ -777,7 +777,7 @@ function(S)
   gens := GeneratorsOfSemigroup(S);
   if Length(gens) <= 1 then
     SetMinimalSemigroupGeneratingSet(S, gens);
-    return true;
+    return Length(gens) = 1;
   elif CanEasilyCompareElements(gens)
       and ForAll([2 .. Length(gens)], i -> gens[1] = gens[i]) then
     SetMinimalSemigroupGeneratingSet(S, [gens[1]]);
diff --git a/tst/standard/properties.tst b/tst/standard/properties.tst
index 87c90e0ff..8eade7aae 100644
--- a/tst/standard/properties.tst
+++ b/tst/standard/properties.tst
@@ -945,6 +945,11 @@ gap> x := MinimalSemigroupGeneratingSet(S)[1];;
 gap> S := Semigroup(x, x, x);;
 gap> IsMonogenicSemigroup(S);
 true
+gap> S := Subsemigroup(FullTransformationMonoid(2), []);;
+gap> IsEmpty(S);
+true
+gap> IsMonogenicSemigroup(S);
+false
 
 # properties: IsMonogenicInverseSemigroup, 1
 gap> IsMonogenicInverseSemigroup(AsSemigroup(IsBooleanMatSemigroup,