Le 14/09/2015 23:04, Francesco Ariis follows my cold remark suggesting that JM should look upon some sources before asking this list to help to solve his homework:

Clue /= solution. Searching the web tentds to give straight answers rather than hints (which some of us find more useful to progress

?

Did YOU, Francesco Ariis, verify the references cited? You reproach me of WHAT?

The StackOverflow page gives plenty of clues, a little theory, data properties, etc.

On the Web you will find what you wish to find. If you don't want hamburger solutions, don't read them. If for you it is more "useful to progress", by querying a community which should be, and IS helpful, but will not replace your ELEMENTARY textbooks, and your teachers, I don't think we share the same vocabulary.

Jerzy Karczmarczuk