module_ m rewrites each function body, folding the sequential type-test chain that Ast_utils.lower_match emits — a run of discarded blocks, each testing the same scrutinee with br_on_cast_fail/br_on_non_null branching to the block's own label, with the arm body after the test — into a high-level match. The statements after the run become the trailing default (sound because each arm body diverges, so they run only on the no-match path). A bound cast arm's separate local declaration (let p : T;) is dropped, since the recovered arm re-declares the binding on re-lowering.

Folding is the exact inverse of Ast_utils.lower_match, so a Wax match round-trips through the binary. Meant to run on Sink_let.module_ output, which places the binding declaration adjacent to its block.