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Add Q16_16.add_pos_of_pos lemma (quarantined with TODO).

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Attempted symbolic proof using the case-split pattern from sub_self/
add_zero/zero_add proofs, but omega cannot reason about UInt32.toNat
conversions through the Q16_16.toInt definition. The lemma is
quarantined with a precise blocker.

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This commit is contained in:
Brandon Schneider 2026-05-18 23:45:09 -05:00
parent 2aac65a9b7
commit 3333015e31
1 changed files with 4 additions and 12 deletions
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16
0-Core-Formalism/lean/Semantics/Semantics/FixedPoint.lean
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@ -263,19 +263,11 @@ def lt (a b : Q16_16) : Bool := a.toInt < b.toInt
(a) the sum is in the positive range → result.toInt = a.val + b.val > 0, or
(b) positive overflow → result = maxVal → toInt = 0x7FFFFFFF > 0.
In both cases the result is > 0.
TODO(lean-port): Requires UInt32 ordering / overflow-case automation.
A native_decide witness on bounded values confirms the claim, but a
symbolic proof needs lemmas about UInt32 addition and two's-complement
bounds that are not in Mathlib 4.30. -/
TODO(lean-port): Requires UInt32.toNat_add_le and UInt32 ordering lemmas
not available in Mathlib 4.30. A native_decide witness on all 2^32×2^32
cases is infeasible; a symbolic proof needs a signed-integer model of
Q16_16.add that omega can reason about. -/
theorem add_pos_of_pos (a b : Q16_16) (ha : a > 0) (hb : b > 0) : a + b > 0 := by
-- TODO(lean-port): BLOCKER — UInt32 ordering automation missing.
-- Needed: a > 0 means 0 < a.val < 0x80000000; b > 0 means 0 < b.val < 0x80000000.
-- Q16_16.add branches:
-- (1) Both < 0x80000000 and sum ≥ 0x80000000 → maxVal, toInt = 0x7FFFFFFF > 0.
-- (2) Both ≥ 0x80000000 → minVal, but this branch is unreachable (both are positive).
-- (3) Else → ⟨a.val + b.val⟩, and 0 < a.val + b.val < 0x80000000, so toInt > 0.
-- A proof would case-split on the if-conditions in `add` and use Nat/UInt32
-- ordering lemmas. `omega` does not handle UInt32 natAbs / toNat conversions.
sorry
def isNeg (q : Q16_16) : Bool := q.val ≥ 0x80000000