Make Random for a (finite) collection take an optional source

This requires moving the function as it was defined too early
to use InstallMethodWithRandomSource

lib/coll.gi

@@ -264,12 +264,17 @@ InstallGlobalFunction( RandomList, function(args...)
 
  return list[Random(source, 1, Length(list))];
 end );
-InstallMethod( Random, "for a (finite) collection",
- [ IsCollection and IsFinite ],
+
+# Use -SUM_FLAGS, as this should always be the last choice method.
+# In particular, we want Random(SomeRandomSource, IsList) to come higher
+# for lists, to avoid an infinite loop.
+InstallMethodWithRandomSource( Random, "for a (finite) collection",
+ [ IsCollection and IsFinite ], -SUM_FLAGS,
  C -> RandomList( Enumerator( C ) ) );
+
 
 RedispatchOnCondition(Random,true,[IsCollection],[IsFinite],0);
-
+RedispatchOnCondition(Random,true,[IsRandomSource,IsCollection],[IsFinite],0);
 
 #############################################################################
 ##

lib/random.gi

@@ -311,3 +311,13 @@ end );
  InstallGlobalFunction("InstallMethodWithRandomSource", func(InstallMethod));
  InstallGlobalFunction("InstallOtherMethodWithRandomSource", func(InstallOtherMethod));
 end)();
+
+InstallMethod( Random, "fallback for GlobalMersenneTwister",
+ [IsMersenneTwister, IsCollection],
+ function(rs, o)
+ if rs = GlobalMersenneTwister then
+ return Random(o);
+ else
+ TryNextMethod();
+ fi;
+end);