S7:-Practice-1.md

@@ -12,9 +12,9 @@
 * [Finish your previous practices](#finish-your-previous-practices)
 * [Exercises](#exercises)
   * [Exercise 1: Creating the Seq1 module](#exercise-1-creating-the-seq1-module)
-  * [Exercise 2: Null sequences](#exercise-2-null-sequences)
-  * [Exercise 3: Null, valid and invalid sequences](#exercise-3-null-valid-and-invalid-sequences)
-  * [Exercise 4: Len() method](#exercise-4-len-method)
+  * [Exercise 2: NULL sequences](#exercise-2-null-sequences)
+  * [Exercise 3: NULL, valid and invalid sequences](#exercise-3-null-valid-and-invalid-sequences)
+  * [Exercise 4: len() method](#exercise-4-len-method)
   * [Exercise 5: count_base() method](#exercise-5-count_base-method)
   * [Exercise 6: count() method](#exercise-6-count-method)
   * [Exercise 7: reverse() method](#exercise-7-reverse-method)