Add constructive recognition for SL_n, take 2

gap/base/kernel.gi

@@ -36,7 +36,10 @@ BindGlobal( "GenerateRandomKernelElementsAndOptionallyVerifyThem",
     verificationSuccess := true;
     # We generate a random element of the kernel as the quotient of a random
     # element and the preimage of its image under the homomorphism.
+    Info(InfoRecog,3, "GenerateRandomKernelElementsAndOptionallyVerifyThem, n = ", n);
+
     for i in [1 .. n] do
+        Info(InfoRecog,3, "GenerateRandomKernelElementsAndOptionallyVerifyThem: ", i, " / ", n);
         # Finding kernel generators and immediate verification must use
         # different random elements! This is ensured by using the same stamp
         # in both situations.

gap/projective/classicalnatural.gi

@@ -654,7 +654,8 @@ RECOG.IsThisSL2Natural := function(gens,f)
       Info(InfoRecog,4,"SL2: Computing stabiliser chain.");
       S := StabilizerChain(Group(gens));
       Info(InfoRecog,4,"SL2: size is ",Size(S));
-      return Size(S) mod (q*(q-1)*(q+1)) = 0;
+      # return Size(S) mod (q*(q-1)*(q+1)) = 0;
+      return Size(S) = (q*(q-1)*(q+1));
   fi;
 
   seenqp1 := false;