RingMapOntoRewrittenResidueClassRing avoids creating a new ring if not necessary

Author: mohamed-barakat

MatricesForHomalg/PackageInfo.g

@@ -5,7 +5,7 @@ PackageName := "MatricesForHomalg",
 Subtitle := "Matrices for the homalg project",
 
 Version := Maximum( [
-  "2019.02.10", ## Mohamed's version
+  "2019.02.12", ## Mohamed's version
 ## this line prevents merge conflicts
   "2019.02.11", ## Fabian's version
 ## this line prevents merge conflicts

MatricesForHomalg/gap/Tools.gi

@@ -7269,9 +7269,12 @@ InstallMethod( RingMapOntoRewrittenResidueClassRing,
 
     zero_rows := ZeroRows( matrix - images );
 
-    indets := indets{zero_rows};
-    
-    S := k * List( indets, String );
+    if Length( indets ) = Length( zero_rows ) then
+        S := A;
+    else
+        indets := indets{zero_rows};
+        S := k * List( indets, String );
+    fi;
 
     if not zero_rows = [ ] then
         map := RingMap( indets, S, R );