Reducing Higher-order Recursion Scheme Equivalence to Coinductive Higher-order Constrained Horn Clauses
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Higher-order constrained Horn clauses (HoCHC) are a semantically-invariant system of higher-order logic modulo theories. With semi-decidable unsolvability over a semi-decidable background theory, HoCHC is suitable for safety verification. Less is known about its relation to larger classes of higher-order verification problems. Motivated by program equivalence, we introduce a coinductive version of HoCHC that enjoys a greatest model property. We define an encoding of higher-order recursion schemes (HoRS) into HoCHC logic programs. Correctness of this encoding reduces decidability of the open HoRS equivalence problem – and, thus, the LambdaY-calculus Böhm tree equivalence problem – to semi-decidability of coinductive HoCHC over a complete and decidable theory of trees.
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