From 6a8cd88535bc687cb2ac4a445f33795ea453c176 Mon Sep 17 00:00:00 2001
From: Max Horn <max@quendi.de>
Date: Fri, 18 Aug 2023 09:20:57 +0200
Subject: [PATCH] Make GroupWithMemory behave more like Group

In particular, ensure that the wrapped generators are made
immutable, and in the case of matrices, possibly converted
to a better representation. This is achieved by calling
`GroupWithGenerators` on the given generators, and then
wrapping the generators of the resulting group.

This avoids a surprising mismatch in behavior between
ordinary groups and groups with memory in some situations.
And of course making the generators immutable prevents
further pitfalls.

Also make sure we have at least one test for GroupWithMemory
where the input is not a group.
---
 lib/memory.gi              | 11 ++++++-----
 tst/testinstall/memory.tst |  2 +-
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/lib/memory.gi b/lib/memory.gi
index d8e85b5367..31f267f516 100644
--- a/lib/memory.gi
+++ b/lib/memory.gi
@@ -123,13 +123,14 @@ InstallGlobalFunction( CopyMemory,
 
 InstallGlobalFunction( GroupWithMemory,
   function(gens)
-    # gens a list of generators
+    # gens a list of generators or a group
     local g,memgens;
-    if IsGroup(gens) then
-        memgens := GeneratorsWithMemory(GeneratorsOfGroup(gens));
-    else
-        memgens := GeneratorsWithMemory(gens);
+    if not IsGroup(gens) then
+        # trick: ensure all transformations that GroupWithGenerators applies
+        # to the generators are applied here, too
+        gens := GroupWithGenerators(gens);
     fi;
+    memgens := GeneratorsWithMemory(GeneratorsOfGroup(gens));
     g := GroupWithGenerators(memgens);
     return g;
   end);
diff --git a/tst/testinstall/memory.tst b/tst/testinstall/memory.tst
index dea81b2c32..dfa6a1116f 100644
--- a/tst/testinstall/memory.tst
+++ b/tst/testinstall/memory.tst
@@ -1,5 +1,5 @@
 #@local G, H, g, h, tmp, stabChain, s1, s2
-gap> G := GroupWithMemory(GroupByGenerators([ (1,2,3,4,5), (1,2) ]));;
+gap> G := GroupWithMemory([ (1,2,3,4,5), (1,2) ]);;
 gap> H := GroupWithMemory(GL(IsMatrixGroup, 3, 3));;
 gap> g := H.1 ^ 2;; h := H.2 ^ 2;;
 gap> StripMemory(g);