diff --git a/CommAlg/StrictSeries.lean b/CommAlg/StrictSeries.lean
new file mode 100644
index 0000000..f294353
--- /dev/null
+++ b/CommAlg/StrictSeries.lean
@@ -0,0 +1,583 @@
+/-
+Copyright (c) 2021 Chris Hughes. All rights reserved.
+Released under Apache 2.0 license as described in the Mathlib file LICENSE.
+Authors: Chris Hughes, Grant Barkley
+-/
+import Mathlib.Order.Lattice
+import Mathlib.Data.List.Sort
+import Mathlib.Logic.Equiv.Fin
+import Mathlib.Logic.Equiv.Functor
+import Mathlib.Data.Fintype.Card
+import Mathlib.Order.Monotone.Basic
+
+structure StrictSeries (X : Type u) [LT X] : Type u where
+  length : ℕ
+  toFun : Fin (length + 1) → X
+  step' : ∀ i : Fin length, (toFun (Fin.castSucc i)) < (toFun (Fin.succ i))
+  --StrictMono toFun
+
+section List
+
+-- TODO: move this to Mathlib.Data.List.Basic
+@[simp]
+theorem List.getLast_tail {X : Type _} {l : List X} {h : l.tail ≠ []} :
+l.tail.getLast h = l.getLast (fun c => (c ▸ h) List.tail_nil) := by
+  cases l
+  . simp
+  . rw [List.getLast_cons]; simp; assumption
+
+end List
+
+namespace StrictSeries
+
+section FinLemmas
+
+-- TODO: move these to `VecNotation` and rename them to better describe their statement
+variable {α : Type _} {m n : ℕ} (a : Fin m.succ → α) (b : Fin n.succ → α)
+
+theorem append_castAdd_aux (i : Fin m) :
+    Matrix.vecAppend (Nat.add_succ _ _).symm (a ∘ Fin.castSucc) b
+      (Fin.castSucc <| Fin.castAdd n i) =
+      a (Fin.castSucc i) := by
+  cases i
+  simp [Matrix.vecAppend_eq_ite, *]
+#align composition_series.append_cast_add_aux StrictSeries.append_castAdd_aux
+
+theorem append_succ_castAdd_aux (i : Fin m) (h : a (Fin.last _) = b 0) :
+    Matrix.vecAppend (Nat.add_succ _ _).symm (a ∘ Fin.castSucc) b (Fin.castAdd n i).succ =
+      a i.succ := by
+  cases' i with i hi
+  simp only [Matrix.vecAppend_eq_ite, hi, Fin.succ_mk, Function.comp_apply, Fin.castSucc_mk,
+    Fin.val_mk, Fin.castAdd_mk]
+  split_ifs with h_1
+  · rfl
+  · have : i + 1 = m := le_antisymm hi (le_of_not_gt h_1)
+    calc
+      b ⟨i + 1 - m, by simp [this]⟩ = b 0 := congr_arg b (by simp [Fin.ext_iff, this])
+      _ = a (Fin.last _) := h.symm
+      _ = _ := congr_arg a (by simp [Fin.ext_iff, this])
+#align composition_series.append_succ_cast_add_aux StrictSeries.append_succ_castAdd_aux
+
+theorem append_natAdd_aux (i : Fin n) :
+    Matrix.vecAppend (Nat.add_succ _ _).symm (a ∘ Fin.castSucc) b
+      (Fin.castSucc <| Fin.natAdd m i) =
+      b (Fin.castSucc i) := by
+  cases i
+  simp only [Matrix.vecAppend_eq_ite, Nat.not_lt_zero, Fin.natAdd_mk, add_lt_iff_neg_left,
+    add_tsub_cancel_left, dif_neg, Fin.castSucc_mk, not_false_iff, Fin.val_mk]
+#align composition_series.append_nat_add_aux StrictSeries.append_natAdd_aux
+
+theorem append_succ_natAdd_aux (i : Fin n) :
+    Matrix.vecAppend (Nat.add_succ _ _).symm (a ∘ Fin.castSucc) b (Fin.natAdd m i).succ =
+      b i.succ := by
+  cases' i with i hi
+  simp only [Matrix.vecAppend_eq_ite, add_assoc, Nat.not_lt_zero, Fin.natAdd_mk,
+    add_lt_iff_neg_left, add_tsub_cancel_left, Fin.succ_mk, dif_neg, not_false_iff, Fin.val_mk]
+#align composition_series.append_succ_nat_add_aux StrictSeries.append_succ_natAdd_aux
+
+end FinLemmas
+
+
+section LT
+
+variable {X : Type u} [LT X]
+
+instance IsEmpty [IsEmpty X] : IsEmpty (StrictSeries X) :=
+  ⟨fun s => IsEmpty.false <| s.toFun 0⟩
+
+instance coeFun : CoeFun (StrictSeries X) fun x => Fin (x.length + 1) → X where
+  coe := StrictSeries.toFun
+
+instance inhabited [Inhabited X] : Inhabited (StrictSeries X) :=
+  ⟨{  length := 0
+      toFun := default
+      step' := fun x => x.elim0 }⟩
+
+instance Nonempty [Nonempty X] : Nonempty (StrictSeries X) :=
+  ⟨{  length := 0
+      toFun := Nonempty.some inferInstance
+      step' := fun x => x.elim0 }⟩
+
+theorem step (s : StrictSeries X) :
+    ∀ i : Fin s.length, (s (Fin.castSucc i)) < (s (Fin.succ i)) :=
+  s.step'
+
+theorem coeFn_mk (length : ℕ) (toFun step) :
+    (@StrictSeries.mk X _ length toFun step : Fin length.succ → X) = toFun :=
+  rfl
+
+theorem lt_succ (s : StrictSeries X) (i : Fin s.length) :
+    s (Fin.castSucc i) < s (Fin.succ i) :=
+   (s.step _)
+
+instance membership : Membership X (StrictSeries X) :=
+  ⟨fun x s => x ∈ Set.range s⟩
+
+theorem mem_def {x : X} {s : StrictSeries X} : x ∈ s ↔ x ∈ Set.range s :=
+  Iff.rfl
+
+/-- The ordered `List X` of elements of a `StrictSeries X`. -/
+def toList (s : StrictSeries X) : List X :=
+  List.ofFn s
+
+/-- Two `StrictSeries` are equal if they are the same length and
+have the same `i`th element for every `i` -/
+theorem ext_fun {s₁ s₂ : StrictSeries X} (hl : s₁.length = s₂.length)
+    (h : ∀ i, s₁ i = s₂ (Fin.cast (congr_arg Nat.succ hl) i)) : s₁ = s₂ := by
+  cases s₁; cases s₂
+  -- Porting note: `dsimp at *` doesn't work. Why?
+  dsimp at hl h
+  subst hl
+  simpa [Function.funext_iff] using h
+
+@[simp]
+theorem length_toList (s : StrictSeries X) : s.toList.length = s.length + 1 := by
+  rw [toList, List.length_ofFn]
+
+theorem toList_ne_nil (s : StrictSeries X) : s.toList ≠ [] := by
+  rw [← List.length_pos_iff_ne_nil, length_toList]; exact Nat.succ_pos _
+
+theorem chain'_toList (s : StrictSeries X) : List.Chain' (· < ·) s.toList :=
+  List.chain'_iff_get.2
+    (by
+      intro i hi
+      simp only [toList, List.get_ofFn]
+      rw [length_toList] at hi
+      exact s.step ⟨i, hi⟩)
+
+@[simp]
+theorem mem_toList {s : StrictSeries X} {x : X} : x ∈ s.toList ↔ x ∈ s := by
+  rw [toList, List.mem_ofFn, mem_def]
+
+/-- Make a `StrictSeries X` from the ordered list of its elements. -/
+def ofList (l : List X) (hl : l ≠ []) (hc : List.Chain' (· < ·) l) : StrictSeries X
+    where
+  length := l.length - 1
+  toFun i :=
+    l.nthLe i
+      (by
+        conv_rhs => rw [← tsub_add_cancel_of_le (Nat.succ_le_of_lt (List.length_pos_of_ne_nil hl))]
+        exact i.2)
+  step' := fun ⟨i, hi⟩ => List.chain'_iff_get.1 hc i hi
+
+theorem length_ofList (l : List X) (hl : l ≠ []) (hc : List.Chain' (· < ·) l) :
+    (ofList l hl hc).length = l.length - 1 :=
+  rfl
+
+theorem ofList_toList (s : StrictSeries X) :
+    ofList s.toList s.toList_ne_nil s.chain'_toList = s := by
+  refine' ext_fun _ _
+  · rw [length_ofList, length_toList, Nat.succ_sub_one]
+  · rintro ⟨i, hi⟩
+    simp [ofList, toList, -List.ofFn_succ]
+
+@[simp]
+theorem ofList_toList' (s : StrictSeries X) :
+    ofList s.toList s.toList_ne_nil s.chain'_toList = s :=
+  ofList_toList s
+
+@[simp]
+theorem toList_ofList (l : List X) (hl : l ≠ []) (hc : List.Chain' (· < ·) l) :
+    toList (ofList l hl hc) = l := by
+  refine' List.ext_get _ _
+  · rw [length_toList, length_ofList,
+      tsub_add_cancel_of_le (Nat.succ_le_of_lt <| List.length_pos_of_ne_nil hl)]
+  · intro i hi hi'
+    dsimp [ofList, toList]
+    rw [List.get_ofFn]
+    rfl
+
+theorem toList_injective : Function.Injective (@StrictSeries.toList X _) :=
+  fun s₁ s₂ (h : List.ofFn s₁ = List.ofFn s₂) => by
+  have h₁ : s₁.length = s₂.length :=
+    Nat.succ_injective
+      ((List.length_ofFn s₁).symm.trans <| (congr_arg List.length h).trans <| List.length_ofFn s₂)
+  have h₂ : ∀ i : Fin s₁.length.succ, s₁ i = s₂ (Fin.cast (congr_arg Nat.succ h₁) i) :=
+    congr_fun <| List.ofFn_injective <| h.trans <| List.ofFn_congr (congr_arg Nat.succ h₁).symm _
+  cases s₁
+  cases s₂
+  dsimp at h h₁ h₂
+  subst h₁
+  simp only [mk.injEq, heq_eq_eq, true_and]
+  simp only [Fin.cast_refl] at h₂
+  exact funext h₂
+
+theorem ext_list {s₁ s₂ : StrictSeries X} (h : toList s₁ = toList s₂) : s₁ = s₂ := 
+  toList_injective h
+
+def ofElement (x : X) : StrictSeries X where
+  length := 0
+  toFun _ := x
+  step' := by simp
+
+@[simp]
+theorem length_ofElement (x : X) :
+    (ofElement x).length = 0 := rfl
+
+@[simp]
+theorem toList_ofElement (x : X) : toList (ofElement x) = [x] := by 
+    obtain ⟨a, ha⟩ := List.length_eq_one.mp (length_ofElement x ▸ length_toList <| ofElement x)
+    have := List.eq_of_mem_singleton <| ha ▸ (mem_toList.mpr ⟨0, rfl⟩ : x ∈ toList (ofElement x))
+    rw [(ha : toList (ofElement x) = _), this]
+
+@[simp]
+theorem mem_ofElement (x : X) {y : X} : y ∈ (ofElement x) ↔ y = x := by
+  rw [←mem_toList, toList_ofElement, List.mem_singleton]
+
+@[simp]
+theorem ofList_singleton {x : X} {hne} {hch} : ofList [x] hne hch = ofElement x := by
+  apply ext_list
+  rw [toList_ofList, toList_ofElement]
+
+theorem length_eq_zero {s : StrictSeries X} :
+  s.length = 0 ↔ ∃ x, s = ofElement x := 
+  ⟨fun h =>  
+    have ⟨a, ha⟩ := List.length_eq_one.mp (h ▸ (length_toList s))
+    ⟨a, by apply ext_list; rw [ha, toList_ofElement]⟩, 
+  fun ⟨x, h⟩ => h.symm ▸ length_ofElement x⟩
+
+theorem ofElement_of_length_zero {s : StrictSeries X} (h : s.length = 0) (hx : x ∈ s) :
+  s = ofElement x := by
+  have ⟨y, hy⟩ := length_eq_zero.mp h
+  -- bug? can't inline this
+  have := mem_ofElement y |>.mp <| hy ▸ hx
+  rwa [this]
+
+/-- The last element of a `StrictSeries` -/
+def top (s : StrictSeries X) : X :=
+  s (Fin.last _)
+
+theorem top_mem (s : StrictSeries X) : s.top ∈ s :=
+  mem_def.2 (Set.mem_range.2 ⟨Fin.last _, rfl⟩)
+
+@[simp]
+theorem ofElement_top {x : X} : (ofElement x).top = x := rfl
+
+@[simp]
+theorem getLast_toList_eq_top (s : StrictSeries X) : s.toList.getLast s.toList_ne_nil = s.top := by
+  erw [List.last_ofFn]; rfl
+
+@[simp]
+theorem top_ofList {l : List X} {hnn} {hcn} : (ofList l hnn hcn).top = l.getLast hnn := by
+  rw [←getLast_toList_eq_top]; simp
+
+theorem length_eq_zero_top {s : StrictSeries X} : s.length = 0 ↔ s = ofElement s.top :=
+  ⟨fun h => ofElement_of_length_zero h (top_mem s), fun h => h.symm ▸ length_ofElement _⟩
+
+/-- The first element of a `StrictSeries` -/
+def bot (s : StrictSeries X) : X :=
+  s 0
+
+theorem bot_mem (s : StrictSeries X) : s.bot ∈ s :=
+  mem_def.2 (Set.mem_range.2 ⟨0, rfl⟩)
+
+@[simp]
+theorem ofElement_bot {x : X} : (ofElement x).bot = x := rfl
+
+theorem length_eq_zero_bot {s : StrictSeries X} : s.length = 0 ↔ s = ofElement s.bot :=
+  ⟨fun h => ofElement_of_length_zero h (bot_mem s), fun h => h.symm ▸ length_ofElement _⟩
+
+/-- Remove the largest element from a `StrictSeries`. If the toFun `s`
+has length zero, then `s.eraseTop = s` -/
+@[simps]
+def eraseTop (s : StrictSeries X) : StrictSeries X
+    where
+  length := s.length - 1
+  toFun i := s ⟨i, lt_of_lt_of_le i.2 (Nat.succ_le_succ tsub_le_self)⟩
+  step' i := by
+    have := s.step ⟨i, lt_of_lt_of_le i.2 tsub_le_self⟩
+    cases i
+    exact this
+
+theorem top_eraseTop (s : StrictSeries X) :
+    s.eraseTop.top = s ⟨s.length - 1, lt_of_le_of_lt tsub_le_self (Nat.lt_succ_self _)⟩ :=
+  show s _ = s _ from
+    congr_arg s
+      (by
+        ext
+        simp only [eraseTop_length, Fin.val_last, Fin.coe_castSucc, Fin.coe_ofNat_eq_mod,
+          Fin.val_mk])
+
+@[simp]
+theorem bot_eraseTop (s : StrictSeries X) : s.eraseTop.bot = s.bot :=
+  rfl
+
+def eraseBot (s : StrictSeries X) : StrictSeries X :=
+  if h : s.length = 0 then s
+  else
+    ofList (s.toList.tail) 
+    (fun hc => h <| s.length_toList ▸ hc ▸ s.toList.length_tail |>.symm) s.chain'_toList.tail
+
+#check Function.invFun
+
+theorem top_eraseBot (s : StrictSeries X) : s.eraseBot.top = s.top :=
+  if h : s.length = 0 then by rw [eraseBot, dif_pos h]
+  else by rw [eraseBot, dif_neg h]; simp
+
+/-- Append two composition toFun `s₁` and `s₂` such that
+the least element of `s₁` is the maximum element of `s₂`. -/
+@[simps length]
+def append (s₁ s₂ : StrictSeries X) (h : s₁.top = s₂.bot) : StrictSeries X where
+  length := s₁.length + s₂.length
+  toFun := Matrix.vecAppend (Nat.add_succ s₁.length s₂.length).symm (s₁ ∘ Fin.castSucc) s₂
+  step' i := by
+    refine' Fin.addCases _ _ i
+    · intro i
+      rw [append_succ_castAdd_aux _ _ _ h, append_castAdd_aux]
+      exact s₁.step i
+    · intro i
+      rw [append_natAdd_aux, append_succ_natAdd_aux]
+      exact s₂.step i
+
+theorem coe_append (s₁ s₂ : StrictSeries X) (h) :
+    ⇑(s₁.append s₂ h) = Matrix.vecAppend (Nat.add_succ _ _).symm (s₁ ∘ Fin.castSucc) s₂ :=
+  rfl
+
+@[simp]
+theorem append_castAdd {s₁ s₂ : StrictSeries X} (h : s₁.top = s₂.bot) (i : Fin s₁.length) :
+    append s₁ s₂ h (Fin.castSucc <| Fin.castAdd s₂.length i) = s₁ (Fin.castSucc i) := by
+  rw [coe_append, append_castAdd_aux _ _ i]
+
+@[simp]
+theorem append_succ_castAdd {s₁ s₂ : StrictSeries X} (h : s₁.top = s₂.bot)
+    (i : Fin s₁.length) : append s₁ s₂ h (Fin.castAdd s₂.length i).succ = s₁ i.succ := by
+  rw [coe_append, append_succ_castAdd_aux _ _ _ h]
+
+@[simp]
+theorem append_natAdd {s₁ s₂ : StrictSeries X} (h : s₁.top = s₂.bot) (i : Fin s₂.length) :
+    append s₁ s₂ h (Fin.castSucc <| Fin.natAdd s₁.length i) = s₂ (Fin.castSucc i) := by
+  rw [coe_append, append_natAdd_aux _ _ i]
+
+@[simp]
+theorem append_succ_natAdd {s₁ s₂ : StrictSeries X} (h : s₁.top = s₂.bot) (i : Fin s₂.length) :
+    append s₁ s₂ h (Fin.natAdd s₁.length i).succ = s₂ i.succ := by
+  rw [coe_append, append_succ_natAdd_aux _ _ i]
+
+/-- Add an element to the top of a `StrictSeries` -/
+@[simps length]
+def snoc (s : StrictSeries X) (x : X) (hsat : s.top < x) : StrictSeries X where
+  length := s.length + 1
+  toFun := Fin.snoc s x
+  step' i := by
+    refine' Fin.lastCases _ _ i
+    · rwa [Fin.snoc_castSucc, Fin.succ_last, Fin.snoc_last, ← top]
+    · intro i
+      rw [Fin.snoc_castSucc, ← Fin.castSucc_fin_succ, Fin.snoc_castSucc]
+      exact s.step _
+#align composition_series.snoc StrictSeries.snoc
+
+@[simp]
+theorem top_snoc (s : StrictSeries X) (x : X) (hsat : s.top < x) :
+    (snoc s x hsat).top = x :=
+  Fin.snoc_last (α := fun _ => X) _ _
+#align composition_series.top_snoc StrictSeries.top_snoc
+
+@[simp]
+theorem snoc_last (s : StrictSeries X) (x : X) (hsat : s.top < x) :
+    snoc s x hsat (Fin.last (s.length + 1)) = x :=
+  Fin.snoc_last (α := fun _ => X) _ _
+#align composition_series.snoc_last StrictSeries.snoc_last
+
+@[simp]
+theorem snoc_castSucc (s : StrictSeries X) (x : X) (hsat : s.top < x)
+    (i : Fin (s.length + 1)) : snoc s x hsat (Fin.castSucc i) = s i :=
+  Fin.snoc_castSucc (α := fun _ => X) _ _ _
+#align composition_series.snoc_cast_succ StrictSeries.snoc_castSucc
+
+@[simp]
+theorem bot_snoc (s : StrictSeries X) (x : X) (hsat : s.top < x) :
+    (snoc s x hsat).bot = s.bot := by rw [bot, bot, ← snoc_castSucc s _ _ 0, Fin.castSucc_zero]
+#align composition_series.bot_snoc StrictSeries.bot_snoc
+
+theorem mem_snoc {s : StrictSeries X} {x y : X} {hsat : s.top < x} :
+    y ∈ snoc s x hsat ↔ y ∈ s ∨ y = x := by
+  simp only [snoc, mem_def]
+  constructor
+  · rintro ⟨i, rfl⟩
+    refine' Fin.lastCases _ (fun i => _) i
+    · right
+      simp
+    · left
+      simp
+  · intro h
+    rcases h with (⟨i, rfl⟩ | rfl)
+    · use Fin.castSucc i
+      simp
+    · use Fin.last _
+      simp
+#align composition_series.mem_snoc StrictSeries.mem_snoc
+
+
+end LT
+
+section Preorder
+
+variable {X : Type _} [Preorder X]
+
+protected theorem strictMono (s : StrictSeries X) : StrictMono s :=
+  Fin.strictMono_iff_lt_succ.2 s.lt_succ
+
+protected theorem injective (s : StrictSeries X) : Function.Injective s :=
+  s.strictMono.injective
+
+@[simp]
+protected theorem inj (s : StrictSeries X) {i j : Fin s.length.succ} : s i = s j ↔ i = j :=
+  s.injective.eq_iff
+
+theorem total {s : StrictSeries X} {x y : X} (hx : x ∈ s) (hy : y ∈ s) : x ≤ y ∨ y ≤ x := by
+  rcases Set.mem_range.1 hx with ⟨i, rfl⟩
+  rcases Set.mem_range.1 hy with ⟨j, rfl⟩
+  rw [s.strictMono.le_iff_le, s.strictMono.le_iff_le]
+  exact le_total i j
+
+theorem toList_sorted (s : StrictSeries X) : s.toList.Sorted (· < ·) :=
+  List.pairwise_iff_get.2 fun i j h => by
+    dsimp [toList]
+    rw [List.get_ofFn, List.get_ofFn]
+    exact s.strictMono h
+
+theorem toList_nodup (s : StrictSeries X) : s.toList.Nodup :=
+  s.toList_sorted.nodup
+
+/-- Two `StrictSeries` on a preorder are equal if they have the same elements. 
+See also `ext_fun` and `ext_list`. -/
+@[ext]
+theorem ext {s₁ s₂ : StrictSeries X} (h : ∀ x, x ∈ s₁ ↔ x ∈ s₂) : s₁ = s₂ :=
+  toList_injective <|
+    List.eq_of_perm_of_sorted
+      (by
+        classical
+        exact List.perm_of_nodup_nodup_toFinset_eq s₁.toList_nodup s₂.toList_nodup
+          (Finset.ext <| by simp [*]))
+      s₁.toList_sorted s₂.toList_sorted
+
+@[simp]
+theorem le_top {s : StrictSeries X} (i : Fin (s.length + 1)) : s i ≤ s.top :=
+  s.strictMono.monotone (Fin.le_last _)
+
+theorem le_top_of_mem {s : StrictSeries X} {x : X} (hx : x ∈ s) : x ≤ s.top :=
+  let ⟨_i, hi⟩ := Set.mem_range.2 hx
+  hi ▸ le_top _
+
+@[simp]
+theorem bot_le {s : StrictSeries X} (i : Fin (s.length + 1)) : s.bot ≤ s i :=
+  s.strictMono.monotone (Fin.zero_le _)
+
+theorem bot_le_of_mem {s : StrictSeries X} {x : X} (hx : x ∈ s) : s.bot ≤ x :=
+  let ⟨_i, hi⟩ := Set.mem_range.2 hx
+  hi ▸ bot_le _
+
+-- TODO this should be in section LT
+theorem length_pos_of_mem_ne {s : StrictSeries X} {x y : X} (hx : x ∈ s) (hy : y ∈ s)
+    (hxy : x ≠ y) : 0 < s.length :=
+  let ⟨i, hi⟩ := hx
+  let ⟨j, hj⟩ := hy
+  have hij : i ≠ j := mt s.inj.2 fun h => hxy (hi ▸ hj ▸ h)
+  hij.lt_or_lt.elim
+    (fun hij => lt_of_le_of_lt (zero_le (i : ℕ)) (lt_of_lt_of_le hij (Nat.le_of_lt_succ j.2)))
+      fun hji => lt_of_le_of_lt (zero_le (j : ℕ)) (lt_of_lt_of_le hji (Nat.le_of_lt_succ i.2))
+
+-- TODO this should be in section LT
+theorem forall_mem_eq_of_length_eq_zero {s : StrictSeries X} (hs : s.length = 0) {x y}
+    (hx : x ∈ s) (hy : y ∈ s) : x = y :=
+  by_contradiction fun hxy => pos_iff_ne_zero.1 (length_pos_of_mem_ne hx hy hxy) hs
+
+theorem eraseTop_top_le (s : StrictSeries X) : s.eraseTop.top ≤ s.top := by
+  simp [eraseTop, top, s.strictMono.le_iff_le, Fin.le_iff_val_le_val, tsub_le_self]
+
+-- TODO this should be in section LT
+theorem mem_eraseTop_of_ne_of_mem {s : StrictSeries X} {x : X} (hx : x ≠ s.top) (hxs : x ∈ s) :
+    x ∈ s.eraseTop := by
+  rcases hxs with ⟨i, rfl⟩
+  have hi : (i : ℕ) < (s.length - 1).succ := by
+    conv_rhs => rw [← Nat.succ_sub (length_pos_of_mem_ne ⟨i, rfl⟩ s.top_mem hx), Nat.succ_sub_one]
+    exact lt_of_le_of_ne (Nat.le_of_lt_succ i.2) (by simpa [top, s.inj, Fin.ext_iff] using hx)
+  refine' ⟨Fin.castSucc i, _⟩
+  simp [Fin.ext_iff, Nat.mod_eq_of_lt hi]
+
+theorem mem_eraseTop {s : StrictSeries X} {x : X} (h : 0 < s.length) :
+    x ∈ s.eraseTop ↔ x ≠ s.top ∧ x ∈ s := by
+  simp only [mem_def]
+  dsimp only [eraseTop]
+  constructor
+  · rintro ⟨i, rfl⟩
+    have hi : (i : ℕ) < s.length := by
+      conv_rhs => rw [← Nat.succ_sub_one s.length, Nat.succ_sub h]
+      exact i.2
+    simp [top, Fin.ext_iff, ne_of_lt hi, -Set.mem_range, Set.mem_range_self]
+  · intro h
+    exact mem_eraseTop_of_ne_of_mem h.1 h.2
+
+theorem lt_top_of_mem_eraseTop {s : StrictSeries X} {x : X} (h : 0 < s.length)
+    (hx : x ∈ s.eraseTop) : x < s.top := by
+    rw [mem_eraseTop h] at hx
+    let ⟨i, hi⟩ := Set.mem_range.2 hx.2
+    rw [←hi]
+    apply s.strictMono
+    apply lt_of_le_of_ne i.le_last
+    intro hc
+    exact ((hc ▸ hi).symm ▸ hx).1 rfl
+    --hi ▸ le_top _
+  -- lt_of_le_of_ne (le_top_of_mem ((mem_eraseTop h).1 hx).2) ((mem_eraseTop h).1 hx).1
+-- #align composition_series.lt_top_of_mem_erase_top StrictSeries.lt_top_of_mem_eraseTop
+
+theorem isMaximal_eraseTop_top {s : StrictSeries X} (h : 0 < s.length) :
+    s.eraseTop.top < s.top := lt_top_of_mem_eraseTop h (top_mem _)
+--   have : s.length - 1 + 1 = s.length := by
+--     conv_rhs => rw [← Nat.succ_sub_one s.length]; rw [Nat.succ_sub h]
+--   rw [top_eraseTop, top]
+--   convert s.step ⟨s.length - 1, Nat.sub_lt h zero_lt_one⟩; ext; simp [this]
+-- #align composition_series.is_maximal_erase_top_top StrictSeries.isMaximal_eraseTop_top
+
+-- TODO should be in LT
+theorem eq_snoc_eraseTop {s : StrictSeries X} (h : 0 < s.length) :
+    s = snoc (eraseTop s) s.top (isMaximal_eraseTop_top h) := by
+  ext x
+  simp [mem_snoc, mem_eraseTop h]
+  by_cases h : x = s.top <;> simp [*, s.top_mem]
+
+-- TODO should be in LT
+@[simp]
+theorem snoc_eraseTop_top {s : StrictSeries X} (h : s.eraseTop.top < s.top) :
+    s.eraseTop.snoc s.top h = s :=
+  have h : 0 < s.length :=
+    Nat.pos_of_ne_zero
+      (by
+        intro hs
+        refine' ne_of_gt h _
+        simp [top, Fin.ext_iff, hs])
+  (eq_snoc_eraseTop h).symm
+
+-- section `Equivalent` doesn't apply here
+
+theorem length_eq_zero_of_bot_eq_bot_of_top_eq_top_of_length_eq_zero {s₁ s₂ : StrictSeries X}
+    (hb : s₁.bot = s₂.bot) (ht : s₁.top = s₂.top) (hs₁ : s₁.length = 0) : s₂.length = 0 := by
+  have : s₁.bot = s₁.top := congr_arg s₁ (Fin.ext (by simp [hs₁]))
+  have : Fin.last s₂.length = (0 : Fin s₂.length.succ) :=
+    s₂.injective (hb.symm.trans (this.trans ht)).symm
+  -- Porting note: Was `simpa [Fin.ext_iff]`.
+  rw [Fin.ext_iff] at this
+  simpa
+
+theorem length_pos_of_bot_eq_bot_of_top_eq_top_of_length_pos {s₁ s₂ : StrictSeries X}
+    (hb : s₁.bot = s₂.bot) (ht : s₁.top = s₂.top) : 0 < s₁.length → 0 < s₂.length :=
+  not_imp_not.1
+    (by
+      simp only [pos_iff_ne_zero, Ne.def, not_iff_not, Classical.not_not]
+      exact length_eq_zero_of_bot_eq_bot_of_top_eq_top_of_length_eq_zero hb.symm ht.symm)
+
+theorem eq_of_bot_eq_bot_of_top_eq_top_of_length_eq_zero {s₁ s₂ : StrictSeries X}
+    (hb : s₁.bot = s₂.bot) (ht : s₁.top = s₂.top) (hs₁0 : s₁.length = 0) : s₁ = s₂ := by
+  have : ∀ x, x ∈ s₁ ↔ x = s₁.top := fun x =>
+    ⟨fun hx => forall_mem_eq_of_length_eq_zero hs₁0 hx s₁.top_mem, fun hx => hx.symm ▸ s₁.top_mem⟩
+  have : ∀ x, x ∈ s₂ ↔ x = s₂.top := fun x =>
+    ⟨fun hx =>
+      forall_mem_eq_of_length_eq_zero
+        (length_eq_zero_of_bot_eq_bot_of_top_eq_top_of_length_eq_zero hb ht hs₁0) hx s₂.top_mem,
+      fun hx => hx.symm ▸ s₂.top_mem⟩
+  ext
+  simp [*]
+
+end Preorder
+
+end StrictSeries