This PR changes the construction of a `CompleteLattice` instance on
predicates (maps intro `Prop`) inside of
`coinductive_fixpoint`/`inductive_fixpoint` machinery.
Consider a following endomap on predicates of the type ` α → Prop`:
```lean4
def DefFunctor (r : α → α → Prop) (infSeq : α → Prop) : α → Prop :=
λ x : α => ∃ y, r x y ∧ infSeq y
```
The following eta-reduced expression failed to elaborate:
```lean4
def def1 (r : α → α → Prop) : α → Prop := DefFunctor r (def1 r)
coinductive_fixpoint monotonicity sorry
```
At the same time, eta-expanded variant would elaborate correctly:
```lean4
def def2 (r : α → α → Prop) : α → Prop := fun x => DefFunctor r (def2 r) x
coinductive_fixpoint monotonicity sorry
```
This PR fixes the above issue, by changing the way how `CompleteLattice`
instance on the space of predicates is constructed, to allow for the
eta-reduced case, as outlined above.
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.
