Lean 4 fork for HoTT-compatible kernel extensions (Path types, transport, HITs). Maintained against upstream leanprover/lean4.
96fcc94acb
This PR adds support for inductive and coinductive predicates defined using lattice theoretic structures on `Prop`. These are syntactically defined using `greatest_fixpoint` or `least_fixpoint` termination clauses for recursive `Prop`-valued functions. The functionality relies on `partial_fixpoint` machinery and requires function definitions to be monotone. For non-mutually recursive predicates, an appropriate (co)induction proof principle (given by Park induction) is generated. Summary of changes: - `Interal.Order.Basic` now contains `CompleteLattice` class, as well as version of Knaster-Tarski fixpoint theorem (with an associated Park induction principle) for the internal use for defining (co)inductive predicates. `Prop` is shown to have two complete lattice structures (one given by implication order for defining inductive predicates, and one given by reverse implication for defining coinductive predicates). Additionally, proofs that lattices are closed under products and function spaces are included. - Partial fixpoint's `EqnInfo` now additionally carries an information whether something is defined as a lattice-theoretic fixpoint or via CCPOs. - When constructing a (co)inductive predicate,`PartialFixpoint/Main` builds an appropriate lattice structure on the type of the predicate using product lattice, function space lattice and an appropriate lattice instance on `Prop`. - `PartialFixpoint/Eqns` is modified to be able to perform rewrite under lattice-theoretic fixpoint construction - `PartialFixpoint/Induction`contains a case split for handling of the (co)inductive predicates. In the case of lattice-theoretic fixpoints, it appropriately desugars the Park induction principle. |
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