Add DAG-based kernel typechecker #300
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Implement a Lean 4 kernel typechecker using a DAG representation with BUBS (Bottom-Up Beta Substitution) for efficient reduction. The kernel operates on a mutable DAG rather than tree-based expressions, enabling in-place substitution and shared subterm reduction. 12 modules: doubly-linked list, DAG nodes with 10 pointer variants, BUBS upcopy with 12 parent cases, Expr/DAG conversion, universe level operations, WHNF via trail algorithm, definitional equality with lazy delta/proof irrelevance/eta, type inference, and checking for quotients and inductives.
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Implement a Lean 4 kernel typechecker using a DAG representation with BUBS (Bottom-Up Beta Substitution) for efficient reduction. The kernel operates on a mutable DAG rather than tree-based expressions, enabling in-place substitution and shared subterm reduction.
12 modules: doubly-linked list, DAG nodes with 10 pointer variants, BUBS upcopy with 12 parent cases, Expr/DAG conversion, universe level operations, WHNF via trail algorithm, definitional equality with lazy delta/proof irrelevance/eta, type inference, and checking for quotients and inductives.