Points to consider
l Variables are bound by Prolog, not by the programmer
You can’t assign a value to a variable.
l Successive user prompts  ; cause the interpreter to return all
terms that can be substituted for X.
They are returned in the order found.
Order is important
l  PROLOG adopts the closed-world assumption:
All knowledge of the world is present in the database.
If a term is not in the database assume is false.
Prolog’s ‘yes’ = I can prove it, ‘no’ = I can’t prove it.
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