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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.

<= (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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