Motivated by applications to proof assistants based on dependent types, we develop and prove correct a strong reducer and βequivalence checker for the λ-calculus with products, sums, and guarded fixpoints. Our approach is based on compilation to the bytecode of an abstract machine performing weak reductions on non-closed terms, derived with minimal modifications from the ZAM machine used in the Objective Caml bytecode interpreter, and complemented by a recursive "read back" procedure. An

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... ntation in the Coq proof assistant demonstrates important speedups compared with the original interpreter-based implementation of strong reduction in Coq.