Make SubdirectProductOp handle mappings which do not know they are ho…

…momorphisms Also introduce a family predicate for SubdirectProductOp for robustness
gap-system · Jun 5, 2019 · d784c25 · d784c25
1 parent ddfb57a
commit d784c25
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Showing 3 changed files with 18 additions and 8 deletions.
diff --git a/lib/gprd.gi b/lib/gprd.gi
@@ -510,19 +510,29 @@ local iso;
     fi;
   fi;
 
-  # the ...Op is installed for `IsGroupHomomorphism'. So we have to enforce
-  # the filter to be set.
-  if not IsGroupHomomorphism(ghom) or not IsGroupHomomorphism(hhom) then
-    Error("mappings are not homomorphisms");
-  fi;
   return SubdirectProductOp(G,H,ghom,hhom);
 end);
 
 #############################################################################
 ##
 #M  SubdirectProduct( <G1>, <G2>, <phi1>, <phi2> )
 ##
-InstallMethod( SubdirectProductOp,"groups", true,
+
+BindGlobal("SubdirectProductFamilyPredicate",{F1, F2, F3, F4} ->
+        IsGeneralMappingFamily(F3) and IsGeneralMappingFamily(F4) and
+        IsIdenticalObj(FamilySource(F3), ElementsFamily(F1)) and
+        IsIdenticalObj(FamilySource(F4), ElementsFamily(F2)) and
+        IsIdenticalObj(FamilyRange(F3), FamilyRange(F4)));
+
+
+RedispatchOnCondition(SubdirectProductOp, "check mappings",
+        SubdirectProductFamilyPredicate,
+        [IsGroup, IsGroup, IsGeneralMapping, IsGeneralMapping],
+        [IsObject, IsObject, IsGroupHomomorphism, IsGroupHomomorphism],
+        10);
+
+
+InstallMethod( SubdirectProductOp,"groups", SubdirectProductFamilyPredicate,
   [ IsGroup, IsGroup, IsGroupHomomorphism, IsGroupHomomorphism ], 0,
 function( G, H, gh, hh )
 local gc,hc,S,info;

diff --git a/lib/gprdpc.gi b/lib/gprdpc.gi
@@ -410,7 +410,7 @@ end);
 ##
 #M  SubdirectProduct( <G1>, <G2>, <phi1>, <phi2> )
 ##
-InstallMethod( SubdirectProductOp,"pcgroup", true,
+InstallMethod( SubdirectProductOp,"pcgroup", SubdirectProductFamilyPredicate,
   [ IsPcGroup, IsPcGroup, IsGroupHomomorphism, IsGroupHomomorphism ], 0,
 function( G, H, gh, hh )
 local pg,ph,kg,kh,ig,ih,mg,mh,S,info;

diff --git a/lib/gprdperm.gi b/lib/gprdperm.gi
@@ -355,7 +355,7 @@ end);
 ##
 #M  SubdirectProduct( <G1>, <G2>, <phi1>, <phi2> )  . . . . . . . constructor
 ##
-InstallMethod( SubdirectProductOp,"permgroup", true,
+InstallMethod( SubdirectProductOp,"permgroup", SubdirectProductFamilyPredicate,
   [ IsPermGroup, IsPermGroup, IsGroupHomomorphism, IsGroupHomomorphism ], 0,
     function( G1, G2, phi1, phi2 )
     local   S,          # subdirect product of <G1> and <G2>, result