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+Require Import Crypto.Algebra.Field.
+Require Import Crypto.Util.Sum Crypto.Util.Prod Crypto.Util.LetIn.
+Require Import Crypto.Util.Decidable.
+Require Import Crypto.Spec.MontgomeryCurve Crypto.Curves.Montgomery.Affine.
+Require Import Crypto.Curves.Montgomery.XZ BinPos.
+
+Module M.
+  Section MontgomeryCurve.
+    Context {F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv}
+            {field:@Algebra.Hierarchy.field F Feq Fzero Fone Fopp Fadd Fsub Fmul Finv Fdiv}
+            {Feq_dec:Decidable.DecidableRel Feq}
+            {char_ge_5:@Ring.char_ge F Feq Fzero Fone Fopp Fadd Fsub Fmul 5}.
+    Local Infix "=" := Feq : type_scope. Local Notation "a <> b" := (not (a = b)) : type_scope.
+    Local Infix "+" := Fadd. Local Infix "*" := Fmul.
+    Local Infix "-" := Fsub. Local Infix "/" := Fdiv.
+    Local Notation "0" := Fzero.  Local Notation "1" := Fone.
+    Local Notation "'∞'" := (inr tt) : core_scope.
+    Local Notation "( x , y )" := (inl (pair x y)).
+
+    Let char_ge_3:@Ring.char_ge F Feq Fzero Fone Fopp Fadd Fsub Fmul (BinNat.N.succ_pos (BinNat.N.two)).
+    Proof. eapply Algebra.Hierarchy.char_ge_weaken; eauto; vm_decide. Qed.
+
+    Context {a b: F} {b_nonzero:b <> 0}.
+    Context {a24:F} {a24_correct:(1+1+1+1)*a24 = a-(1+1)}.
+    Local Notation add := (M.add(a:=a)(b_nonzero:=b_nonzero)(char_ge_3:=char_ge_3)).
+    Local Notation opp := (M.opp(a:=a)(b_nonzero:=b_nonzero)).
+    Local Notation point := (@M.point F Feq Fadd Fmul a b).
+    Local Notation xzladderstep := (M.xzladderstep(a24:=a24)(Fadd:=Fadd)(Fsub:=Fsub)(Fmul:=Fmul)).
+
+    Ltac t :=
+      repeat
+        match goal with
+        | _ => solve [ contradiction | trivial ]
+        | _ => progress intros
+        | _ => progress subst
+        | _ => progress Tactics.DestructHead.destruct_head' @M.point
+        | _ => progress Tactics.DestructHead.destruct_head' @prod
+        | _ => progress Tactics.DestructHead.destruct_head' @sum
+        | _ => progress Tactics.DestructHead.destruct_head' @and
+        | _ => progress Sum.inversion_sum
+        | _ => progress Prod.inversion_prod
+        | _ => progress Tactics.BreakMatch.break_match_hyps
+        | _ => progress Tactics.BreakMatch.break_match
+        | _ => progress cbv [fst snd M.coordinates M.add M.zero M.eq M.opp proj1_sig M.xzladderstep M.to_xz Let_In] in *
+        | |- _ /\ _ => split
+        end.
+
+    Lemma xzladderstep_correct
+          (Q Q':point) x z x' z' x1 x2 z2 x3 z3
+          (Hl:Logic.eq (pair(pair x2 z2)(pair x3 z3)) (xzladderstep x1 (pair x z) (pair x' z')))
+          (H:match M.coordinates Q with∞=>z=0/\x<>0|(xQ,y)=>xQ=x/z/\z<>0  (* TODO *) /\ y <> 0 (* TODO: prove this from non-squareness of a^2 - 4 *) end)
+          (H':match M.coordinates Q' with∞=>z'=0/\x'<>0|(xQ',_)=>xQ'=x'/z'/\z'<>0 end)
+          (H1:match M.coordinates (add Q (opp Q')) with∞=>False|(x,y)=>x=x1/\x<>0 end):
+      match M.coordinates (add Q Q) with∞=>z2=0/\x2<>0|(xQQ,_)=>xQQ=x2/z2/\z2<>0 end /\
+      match M.coordinates (add Q Q') with∞=>z3=0/\x3<>0|(xQQ',_)=>xQQ'=x3/z3/\z3<>0 end.
+    Proof using a24_correct char_ge_5. t; abstract fsatz. Qed.
+  End MontgomeryCurve.
+End M.