diff --git a/lectures/12_clean_code.md b/lectures/12_clean_code.md
index 3c5e79e..276bf75 100644
--- a/lectures/12_clean_code.md
+++ b/lectures/12_clean_code.md
@@ -1041,122 +1041,6 @@ def fizzbuzz(i: int) -> str:
 :::
 
 
-# denoting blocks
-
-:::::::::::: {.columns .column-gapless}
-::::::::: {.column width="50"}
-```javascript
-for (i = 0; i < 10; i++) {
-    console.log(i);
-}
-```
-```javascript
-for (i = 0; i < 10; i++)
-    console.log(i);
-
-```
-```javascript
-var a = 0;
-for (i = 0; i < 10; i++)
-    a++;
-    console.log(i);
-
-```
-
-:::::::::
-::::::::: {.column width="25%"}
-
-::: {.fragment data-fragment-index="1"}
-```python
-for i in range(10):
-    print(i)
-```
-```python
-a = 0
-for i in range(10):
-    a += 1
-    print(i)
-```
-
-:::
-:::::::::
-::::::::: {.column width="25%"}
-::: {.fragment data-fragment-index="2"}
-<!--```ruby
-a = 0
-for i in 0..9 do
-   a += 1
-   puts i
-end
-```
-```ruby
-a = 0
-for i in 0..9 {
-   a += 1
-   puts i
-}
-```-->
-```go
-package main
- 
-import (
-    "fmt"
-)
- 
-func main() {
-    for i:=0; i<10; i++ {
-        fmt.Println(i)
-    }
-}
-```
-
-```rust
-fn main() {
-    for i in 0..9 {
-        println!("{}", i);
-    }
-}
-```
-:::
-
-:::::::::
-::::::::::::
-
-
-## what could go wrong?
-
-:::::: {.fragment}
-::: {.text-smaller}
-parts from [sslKeyExchange.c](https://opensource.apple.com/source/Security/Security-55471/libsecurity_ssl/lib/sslKeyExchange.c?txt)
-:::
-
-```c {.numberLines startFrom="623"}
-if ((err = ReadyHash(&SSLHashSHA1, &hashCtx)) != 0)
-    goto fail;
-if ((err = SSLHashSHA1.update(&hashCtx, &clientRandom)) != 0)
-    goto fail;
-if ((err = SSLHashSHA1.update(&hashCtx, &serverRandom)) != 0)
-    goto fail;
-if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0)
-    goto fail;
-    goto fail;
-if ((err = SSLHashSHA1.final(&hashCtx, &hashOut)) != 0)
-    goto fail;
-```
-
-```c {.numberLines startFrom="647"}
-fail:
-    SSLFreeBuffer(&signedHashes);
-    SSLFreeBuffer(&hashCtx);
-    return err;
-```
-
-::: {.fragment}
-more about Apple's "goto fail" fiasco (2014): [@wheeler2014apple], [@migues2014understanding]
-:::
-::::::
-
-
 # references
 
 ::: {#refs}
diff --git a/lectures/13_code_quality.md b/lectures/13_code_quality.md
new file mode 100644
index 0000000..7653830
--- /dev/null
+++ b/lectures/13_code_quality.md
@@ -0,0 +1,129 @@
+---
+title: code quality
+author: Gergő Pintér, PhD
+date: gergo.pinter@uni-corvinus.hu
+lang: en-US
+title-slide-attributes:
+    data-background-color: "#181d37"
+    data-background-image: ../assets/by-sa.svg
+    data-background-size: 125px
+    data-background-position: 1.25rem calc(100% - 1.25rem)
+slideNumber: "true"
+showSlideNumber: "print"
+revealjs-url: "../assets/reveal.js-5.1.0/"
+---
+
+# denoting blocks
+
+:::::::::::: {.columns .column-gapless}
+::::::::: {.column width="50"}
+```javascript
+for (i = 0; i < 10; i++) {
+    console.log(i);
+}
+```
+```javascript
+for (i = 0; i < 10; i++)
+    console.log(i);
+
+```
+```javascript
+var a = 0;
+for (i = 0; i < 10; i++)
+    a++;
+    console.log(i);
+
+```
+
+:::::::::
+::::::::: {.column width="25%"}
+
+::: {.fragment data-fragment-index="1"}
+```python
+for i in range(10):
+    print(i)
+```
+```python
+a = 0
+for i in range(10):
+    a += 1
+    print(i)
+```
+
+:::
+:::::::::
+::::::::: {.column width="25%"}
+::: {.fragment data-fragment-index="2"}
+<!--```ruby
+a = 0
+for i in 0..9 do
+   a += 1
+   puts i
+end
+```
+```ruby
+a = 0
+for i in 0..9 {
+   a += 1
+   puts i
+}
+```-->
+```go
+package main
+ 
+import (
+    "fmt"
+)
+ 
+func main() {
+    for i:=0; i<10; i++ {
+        fmt.Println(i)
+    }
+}
+```
+
+```rust
+fn main() {
+    for i in 0..9 {
+        println!("{}", i);
+    }
+}
+```
+:::
+
+:::::::::
+::::::::::::
+
+
+## what could go wrong?
+
+:::::: {.fragment}
+::: {.text-smaller}
+parts from [sslKeyExchange.c](https://opensource.apple.com/source/Security/Security-55471/libsecurity_ssl/lib/sslKeyExchange.c?txt)
+:::
+
+```c {.numberLines startFrom="623"}
+if ((err = ReadyHash(&SSLHashSHA1, &hashCtx)) != 0)
+    goto fail;
+if ((err = SSLHashSHA1.update(&hashCtx, &clientRandom)) != 0)
+    goto fail;
+if ((err = SSLHashSHA1.update(&hashCtx, &serverRandom)) != 0)
+    goto fail;
+if ((err = SSLHashSHA1.update(&hashCtx, &signedParams)) != 0)
+    goto fail;
+    goto fail;
+if ((err = SSLHashSHA1.final(&hashCtx, &hashOut)) != 0)
+    goto fail;
+```
+
+```c {.numberLines startFrom="647"}
+fail:
+    SSLFreeBuffer(&signedHashes);
+    SSLFreeBuffer(&hashCtx);
+    return err;
+```
+
+::: {.fragment}
+more about Apple's "goto fail" fiasco (2014): [@wheeler2014apple], [@migues2014understanding]
+:::
+::::::