diff --git a/src/Init/Data/BitVec/Lemmas.lean b/src/Init/Data/BitVec/Lemmas.lean
index 1cf870d30dfb..da9c19021214 100644
--- a/src/Init/Data/BitVec/Lemmas.lean
+++ b/src/Init/Data/BitVec/Lemmas.lean
@@ -619,6 +619,8 @@ theorem le_toInt {w : Nat} {x : BitVec w} : -2 ^ w ≤ 2 * x.toInt := by
   · rw [← Nat.two_pow_pred_add_two_pow_pred (by omega), ← Nat.two_mul, Nat.add_sub_cancel]
     simp only [Nat.zero_lt_succ, Nat.mul_lt_mul_left, Int.natCast_mul, Int.Nat.cast_ofNat_Int]
     norm_cast; omega
+@[simp] theorem toInt_cast (h : w = v) (x : BitVec w) : (cast h x).toInt = x.toInt := by
+  simp [toInt_eq_toNat_bmod, h]
 
 /-! ### slt -/
 
@@ -673,6 +675,12 @@ theorem zeroExtend_eq_setWidth {v : Nat} {x : BitVec w} :
     (x.setWidth v).toFin = Fin.ofNat' (2^v) x.toNat := by
   ext; simp
 
+theorem setWidth'_eq {x : BitVec w} (h : w ≤ v) : x.setWidth' h = x.setWidth v := by
+  apply eq_of_toNat_eq
+  rw [toNat_setWidth, toNat_setWidth']
+  rw [Nat.mod_eq_of_lt]
+  exact Nat.lt_of_lt_of_le x.isLt (Nat.pow_le_pow_right (Nat.zero_lt_two) h)
+
 @[simp] theorem setWidth_eq (x : BitVec n) : setWidth n x = x := by
   apply eq_of_toNat_eq
   let ⟨x, lt_n⟩ := x
@@ -2026,6 +2034,23 @@ theorem append_def (x : BitVec v) (y : BitVec w) :
     (x ++ y).toNat = x.toNat <<< n ||| y.toNat :=
   rfl
 
+/-- Helper theorem to show that the expression in `(x ++ y).toFin` is inbounds. -/
+theorem toNat_append_lt {m n : Nat} (x : BitVec m) (y : BitVec n) :
+    x.toNat <<< n ||| y.toNat < 2 ^ (m + n) := by
+  have hnLe : 2^n ≤ 2 ^(m + n) := by
+    rw [Nat.pow_add]
+    exact Nat.le_mul_of_pos_left (2 ^ n) (Nat.two_pow_pos m)
+  apply Nat.or_lt_two_pow
+  · have := Nat.two_pow_pos n
+    rw [Nat.shiftLeft_eq, Nat.pow_add, Nat.mul_lt_mul_right]
+    <;> omega
+  · omega
+
+@[simp] theorem toFin_append {x : BitVec m} {y : BitVec n} :
+    (x ++ y).toFin = @Fin.mk (2^(m+n)) (x.toNat <<< n ||| y.toNat) (toNat_append_lt x y) := by
+  ext
+  simp
+
 theorem getLsbD_append {x : BitVec n} {y : BitVec m} :
     getLsbD (x ++ y) i = if i < m then getLsbD y i else getLsbD x (i - m) := by
   simp only [append_def, getLsbD_or, getLsbD_shiftLeftZeroExtend, getLsbD_setWidth']
@@ -2072,6 +2097,173 @@ theorem msb_append {x : BitVec w} {y : BitVec v} :
   ext
   simp [getElem_append]
 
+theorem append_zero {n m : Nat} {x : BitVec n} :
+    x ++ 0#m = x.signExtend (n + m) <<< m := by
+  induction m
+  case zero =>
+    simp [signExtend]
+  case succ i ih =>
+    simp [bv_toNat]
+    sorry
+
+def lhs (x : BitVec n) (y : BitVec m) : Int := (x++y).toInt
+def rhs (x : BitVec n) (y : BitVec m) : Int := if n == 0 then y.toInt else (x.toInt * (2^m)) + y.toNat
+
+def eq (x: BitVec n) (y: BitVec m) : Bool := (lhs x y) = (rhs x y)
+
+
+#eval (-5#10 ++ 3#2).toInt
+
+def test : Bool := Id.run do
+  for i in [0, 1, 2, 3, 4, 5, 6, 7, 8] do
+    for j in [0, 1, 2, 3, 4, 5, 6, 7, 8] do
+      for n in [0, 1, 2, 3, 4] do
+        for m in [0, 1, 2, 3, 4] do
+          let x := BitVec.ofNat n i
+          let y := BitVec.ofNat m j
+          if (!eq x y) then
+            return false
+  return true
+
+private theorem Nat.lt_mul_of_le_of_lt_of_lt {a b c : Nat} (hab : a ≤ b) (ha : 0 < a) (hc : 1 < c) :
+    a < b * c := by
+  have : a * 1 < b * c := Nat.mul_lt_mul_of_le_of_lt' (by omega) (by simp [hc]) (by omega)
+  simp at this
+  simp [this]
+
+private theorem Nat.two_pow_lt_two_pow_add {n m : Nat} (h : m ≠ 0) :
+    2 ^ n < 2 ^ (n + m) := by
+  apply Nat.pow_lt_pow_of_lt (by omega) (by omega)
+
+@[simp] theorem signExtend_shiftLeft_msb {n m : Nat} {x : BitVec n} :
+    (signExtend (n + m) x <<< m).msb = x.msb := by
+  induction m
+  case zero =>
+    simp [signExtend]
+  case succ i ih =>
+    rw [← ih]
+    rw [msb_setWidth]
+
+    unfold BitVec.msb getMsbD
+    simp
+    by_cases h : (0 < n + i)
+    ·
+      rw [← Nat.add_assoc]
+      simp [h]
+      have h' : (0 < n + i + 1) := by omega
+      have hh : (n + i - (i + 1)) = (n + i - i - 1) := by
+        omega
+      rw [hh]
+      simp
+      have hhh : (n + i - 1 - i) = (n + i - i - 1) := by omega
+      rw [hhh]
+      simp
+      rw [getLsbD_signExtend]
+
+
+
+
+
+
+    simp [BitVec.msb, getMsbD]
+
+    by_cases h : 0 < n + (i + 1)
+    · simp [h]
+
+      sorry
+    · simp [h]
+      sorry
+
+@[simp] theorem signExtend_toNat_shift_mod :
+    ((signExtend (n + m) x).toNat <<< m) % ↑(2 ^ (n + m)) = (signExtend (n + m) x).toNat <<< m :=
+  sorry
+
+@[simp] theorem toInt_append_zero {n m : Nat} {x : BitVec n} :
+    (x ++ 0#m).toInt = x.toInt * (2 ^ m) := by
+  by_cases m0 : m = 0
+  · subst m0
+    simp
+  · simp only [ofNat_eq_ofNat, append_zero, toInt_eq_msb_cond]
+    by_cases h1 : (signExtend (n + m) x <<< m).msb
+    · by_cases h2: x.msb
+      · norm_cast
+        simp [h1, h2]
+        norm_cast
+        rw [Int.sub_mul, Nat.pow_add]
+        norm_cast
+        simp
+        rw [Nat.shiftLeft_eq]
+        norm_cast
+        have aa := @Nat.pow_pos 2 m (by omega)
+        norm_cast
+        have bb := @Nat.mul_right_cancel_iff (2^m) ((signExtend (n + m) x).toNat)
+        apply bb
+        rfl
+        rw [Nat.mul_right_cancel (m := 2 ^ m)]
+        simp [aa]
+        rw [Nat.mod_eq_of_lt (a := x.toNat) (by omega)]
+        norm_cast
+        simp [h3]
+        simp_all
+        rw [Nat.shiftLeft_eq]
+      · simp only [signExtend_shiftLeft_of_lt] at h1
+        contradiction
+    · by_cases h2: x.msb
+      · simp [signExtend_shiftLeft_of_lt, h2] at h1
+      · sorry
+
+@[simp] theorem toInt_append {x : BitVec n} {y : BitVec m} :
+    (x ++ y).toInt = if n == 0 then y.toInt else x.toInt * (2 ^ m) + y.toNat := by
+  by_cases n0 : n = 0
+  · subst n0
+    simp [BitVec.eq_nil x]
+  · by_cases m0 : m = 0
+    · subst m0
+      simp [BitVec.eq_nil y, n0]
+    · simp [m0]
+      by_cases y0 : y = 0
+      · simp [toInt_append_zero, y0, n0]
+        rw [toInt_eq_toNat_cond]
+        rw [toInt_eq_toNat_cond]
+        split
+        ·
+          split
+          <;> norm_cast
+          <;> simp
+          <;> rw [Nat.mod_eq_of_lt (a := x.toNat) (by omega)]
+          <;> norm_cast
+          <;> simp [h3]
+          <;> simp_all
+          · rw [Nat.shiftLeft_eq]
+          · rename_i aa bb
+            rw [Nat.shiftLeft_eq] at aa
+            rw [Nat.pow_add] at aa
+            rw [← Nat.mul_assoc] at aa
+
+            sorry
+        ·
+          split
+          <;> norm_cast
+          <;> simp
+          <;> rw [Nat.mod_eq_of_lt (a := x.toNat) (by omega)]
+          <;> norm_cast
+          <;> simp [h3]
+          <;> simp_all
+          · rename_i aa bb
+            rw [Nat.shiftLeft_eq] at aa
+            rw [Nat.pow_add] at aa
+            rw [← Nat.mul_assoc] at aa
+
+
+
+
+            simp_all
+
+
+            sorry
+          · simp [Nat.shiftLeft_eq, Int.sub_mul, Nat.pow_add]
+      · sorry
+
 @[simp] theorem cast_append_right (h : w + v = w + v') (x : BitVec w) (y : BitVec v) :
     (x ++ y).cast h = x ++ y.cast (by omega) := by
   ext