Dear Rasha Kamil, Sir,

Perhaps, by leaving the answer/link blank, Robert is inferring that the backward chaining algorithm returns the value of ZERO ??? *hahahhahaha* ... No, I joke, I am sure that his attempt to copy the link/URL failed for some reason.

Try these links to papers that provide simple/straight-forward explanations of the algorithm:

a scholarly paper as relates to artificial intelligence is here:

http://www.sciencedirect.com/science/article/pii/S0004370206000543/pdf?md5=325f1213c17ec17d6f99374a85079737&pid=1-s2.0-S0004370206000543-main.pdf

a simple paper as relates to computer programming is here:

http://homepage.cs.uri.edu/~cingiser/csc481/chapter_notes/amarant.pdf

Mr. Woo explains in a good lecture (with very good English language and clear mathematics) on YouTube:

https://www.youtube.com/watch?v=ZhTt-GG7PiQ

The VERY basic backward chaining algorithm/theory examined on Wikipedia here:

http://en.wikipedia.org/wiki/Backward_chaining

Regards,

Bob Skiles