Is Horn clause logic decidable?

[Daniel Schwartz]

I have been told that Horn Clause Logic, considered as a subsystem of
First-Order Logic, is undecidable.  More exactly, if S is a set of Horn
clauses and P is an atom, then whether P is derivable from S is undecidable.

However, I have never seen a proof of this and have been unable to find any
discussion of it.

Would anyone know whether this statemnt is true, and could you provide a
refernce?

Thanks and best regards,

Dan Schwartz

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