Coders have not changed mush for the past 0 years. Test-driven-development There are rules for that

4. 
   lesson.
   

1. 
   Coders have not changed mush for the past 50 years.
   
2. 
   Test-driven-development There are 3 rules for that.
   

1. 
   Create a line of test code for not existing production code
   
2. 
   Don’t create another line of test code before previous one compiled.
   
3. 
   Don’t write production code before that test was passed
   

3. 
   Mutation testing that is kind of test runs all test if passed you must change return type from int to long something like that. If mutation testing is still alive after changing type of return value. The code must be refactored. Mutation testing is expensive and it uses PyTest library in Java language.
   
4. 
   Exploritory testing