* 2020-05-27
** note
   - ex27.py
     - none
   - ex28.py
     - <= (greater than or equal to), =< (less than or equal t0)
   - ex29.py
     - The 'if' means to test criteria.
     - The 'if' processes the script is intended.
     - If the script is not indented, an error should be occured.
   - ex30.py
     - none
   - ex31.py
     - none
* 2020-05-25
** note
   - ex23.py
     - none
   - ex24.py
     - none
   - ex25.py
     - none
   - ex26.py
     - none
* 2020-05-22
** note
   - ex15.py
     - The argument method is better than using stdin. becouse bash-completion could be used.
     - python3.8 shell
       - >>> filename=input()
       - ex15_sample.txt
       - >>> txt = open(filename)
       - >>> txt_script = txt.read()
       - >>> print(txt_script)
       - This is stuff I typed into a file.
       - It is really cool stuff.
       - Lots and lots of fun to have in here
   - ex16.py
     - 'w' is to open a file on write mode.
     - If a file is openned with 'w', target.truncate() should not be need. the 'w' mode always overwrite a file.
   - ex17.py
     - cat is concatenate files to stdout
     - If file.close() is not called, the file which is openned by python chould not be modified on orther programs.
   - ex18.py
     - none
   - ex19.py
     - none
   - ex20.py
     - file.seek is to move to new file posotion
   - ex21.py
     - none
   - ex22.py
     - none
* 2020-05-20
** note
   - ex8.py
     - none
   - ex9.py
     - none
   - ex10.py
     - none
   - ex11.py
     - none
   - ex12.py
     - pydoc input
       - describe input on python. It looks like man commnad on linux shell.
   - ex13.py
     - none
   - ex14.py
     - none
* 2020-05-18
** note
   - ex1.py
     - # is commnet.
   - ex2.py
     - I do't find mistakes.
   - ex3.py
     - none
   - ex4.py
     - car_pool_capacity is not defined.
   - ex5.py
     - none
   - ex6.py
     - + is to add two string array.
   - ex7.py
     - none