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Prolog (Programming in Logic) is a programming language based on Predicate Logic. It uses the restricted syntax of Horn Clauses:
Predicates, e.g., floats(), etc.
Operators <=, and, not
Logical variables X, GP, etc.
Constants 7, girl, etc.
Function names f, left(), ...
Facts female(girl).
rules burns(X) <= wooden(X).
queries ? witch(girl).
Quantification of logical variables is implicit -- universal for facts and rules, existential for queries.
The introduction will get you going and there are more examples. The interpreter shows how Prolog works.
NB. The applet above needs Java on.
<= (also :- ) can be read as if, or as is implied by.
planet(P)<=orbits(P,sun)  can be read as ∀ P, P is a planet if P orbits the sun. (That is not to say that the definition cannot be tightened.)
?satellite(S) can be read as does there exist an S such that S is a satellite?
And the answer, here, is yes, if S is earth's moon, or a moon of another planet.

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