diff --git a/.github/workflows/test-dafny-all.yml b/.github/workflows/test-dafny-all.yml
index e7c7d86..06634bf 100644
--- a/.github/workflows/test-dafny-all.yml
+++ b/.github/workflows/test-dafny-all.yml
@@ -13,5 +13,7 @@ jobs:
         uses: cachix/install-nix-action@v20
       - name: Test that all dafny files are named correctly
         run: nix run .#dafny-namecheck
+      - name: Test that all dafny files contain an impl
+        run: nix run .#check-contains-impl
       - name: Run Dafny on all files
         run: nix run .#dafny-check
diff --git a/.github/workflows/test-dafny-new.yml b/.github/workflows/test-dafny-new.yml
index d717c0a..ac0a2f7 100644
--- a/.github/workflows/test-dafny-new.yml
+++ b/.github/workflows/test-dafny-new.yml
@@ -16,6 +16,8 @@ jobs:
         uses: tj-actions/changed-files@v45
       - name: Test that all dafny files are named correctly
         run: nix run .#dafny-namecheck
+      - name: Test that all dafny files contain an impl
+        run: nix run .#check-contains-impl
 
       - name: Run Dafny on new files
         env:
diff --git a/023-strlen.dfy b/023-strlen.dfy
index c380e30..478cd56 100644
--- a/023-strlen.dfy
+++ b/023-strlen.dfy
@@ -1,5 +1,9 @@
 function strlen(s: string) : (len: int)
+  // post-conditions-start
   ensures len == |s|
+  // post-conditions-end
 {
+  // impl-start
   |s|
+  // impl-end
 }
diff --git a/025-factorize.dfy b/025-factorize.dfy
index ba4f214..6ea82bc 100644
--- a/025-factorize.dfy
+++ b/025-factorize.dfy
@@ -3,31 +3,40 @@ function prod(s: seq<int>) : int {
 }
 
 method factorize(n: nat) returns (factors: seq<nat>)
+  // pre-conditions-start
   requires n > 0
+  // pre-conditions-end
+  // post-conditions-start
   ensures prod(factors) == n
+  // post-conditions-end
 {
+  // impl-start
   factors := [];
   ghost var taken := 1;
   var cur := n;
   var i := 2;
   while i * i <= cur
+    // invariants-start
     invariant prod(factors) == taken
     invariant taken * cur == n
     invariant cur >= 1
+    // invariants-end
   {
     ghost var pre := cur;
     ghost var temp := 1;
     while cur % i == 0 
+      // invariants-start
       invariant cur >= 1
       invariant temp * cur == pre
       invariant prod(factors) == taken * temp 
+      // invariants-end
       decreases cur - 1
     {
       factors := factors + [i];
       
       cur := cur / i;
       temp := temp * i;
-      assert 2 <= i && 2 * cur <= i * cur;
+      assert 2 <= i && 2 * cur <= i * cur; // assert-line
     }
     taken := taken * temp;
     i := i + 1;
@@ -36,5 +45,6 @@ method factorize(n: nat) returns (factors: seq<nat>)
     factors := factors + [cur];
     taken := taken * cur;
   }
-  assert taken == n;
+  assert taken == n; // assert-line
+  // impl-end
 }
\ No newline at end of file
diff --git a/flake.nix b/flake.nix
index 8572ea8..4608228 100644
--- a/flake.nix
+++ b/flake.nix
@@ -24,6 +24,23 @@
             done
           '';
 
+          check-contains-impl = pkgs.writeShellScriptBin "check-contains-impl" ''
+            DIR=''${1:-.}
+
+            for file in "$DIR"/*.dfy; do
+              if [[ -e $file ]]; then
+                filename=$(basename "$file")
+
+                if ! ${pkgs.gnugrep}/bin/grep -q "// impl-start" "$file"; then
+                  echo "File $file does not contain an impl, please recheck if it was marked"
+                  exit 1
+                fi
+              fi
+            done
+
+            echo "All dafny files contain an impl."
+          '';
+
           dafny-check-new = pkgs.writeShellScriptBin "dafny-check" ''
             file_count=0