Try this: simp only [f]
[Meta.Tactic.simp.rewrite] unfold f, f (a :: b = []) ==> a :: b = []
[Meta.Tactic.simp.rewrite] @eq_self:1000, False = False ==> True
Try this: simp only [length]
[Meta.Tactic.simp.rewrite] unfold length, length (a :: b :: as) ==> length (b :: as) + 1
[Meta.Tactic.simp.rewrite] unfold length, length (b :: as) ==> length as + 1
Try this: simp only [fact]
[Meta.Tactic.simp.rewrite] unfold fact, fact (Nat.succ x) ==> (x + 1) * fact x
Try this: simp only [head]
[Meta.Tactic.simp.rewrite] unfold head, head (a :: as) ==> match a :: as with
    | [] => default
    | a :: tail => a
[Meta.Tactic.simp.rewrite] @eq_self:1000, a = a ==> True
Try this: simp only [foo]
[Meta.Tactic.simp.rewrite] unfold foo, foo ==> 10
[Meta.Tactic.simp.rewrite] @eq_self:1000, 10 + x = 10 + x ==> True
Try this: simp only [g, Id.pure_eq]
[Meta.Tactic.simp.rewrite] unfold g, g x ==> Id.run
      (let x := x;
      pure x)
[Meta.Tactic.simp.rewrite] @Id.pure_eq:1000, pure x ==> x
Try this: simp only [f1, modify, modifyGet, MonadStateOf.modifyGet, StateT.modifyGet, pure, f2, bind, StateT.bind, get,
  getThe, MonadStateOf.get, StateT.get, set, StateT.set]
[Meta.Tactic.simp.rewrite] unfold f1, f1 ==> modify fun x => g x
[Meta.Tactic.simp.rewrite] unfold modify, modify fun x => g x ==> modifyGet fun s => (PUnit.unit, (fun x => g x) s)
[Meta.Tactic.simp.rewrite] unfold StateT.modifyGet, StateT.modifyGet fun s =>
      (PUnit.unit, (fun x => g x) s) ==> fun s => pure ((fun s => (PUnit.unit, (fun x => g x) s)) s)
[Meta.Tactic.simp.rewrite] unfold f2, f2 ==> do
      let s ← get
      set (g s)
[Meta.Tactic.simp.rewrite] unfold StateT.bind, StateT.bind get fun s => set (g s) ==> fun s => do
      let __discr ← get s
      match __discr with
        | (a, s) => (fun s => set (g s)) a s
[Meta.Tactic.simp.rewrite] unfold getThe, getThe Nat s ==> MonadStateOf.get s
[Meta.Tactic.simp.rewrite] unfold StateT.get, StateT.get s ==> pure (s, s)
[Meta.Tactic.simp.rewrite] unfold StateT.set, StateT.set (g a) s ==> pure (PUnit.unit, g a)
[Meta.Tactic.simp.rewrite] @eq_self:1000, (fun s => (PUnit.unit, g s)) = fun s => (PUnit.unit, g s) ==> True
Try this: simp only [bla, h]
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
[Meta.Tactic.simp.rewrite] @eq_self:1000, x + x = x + x ==> True
Try this: simp only [h, Nat.sub_add_cancel]
[Meta.Tactic.simp.rewrite] h:1000, 1 ≤ x ==> True
[Meta.Tactic.simp.rewrite] @Nat.sub_add_cancel:1000, x - 1 + 1 ==> x
[Meta.Tactic.simp.rewrite] @eq_self:1000, x + 2 = x + 2 ==> True
Try this: simp (config := { contextual := true }) only [Nat.sub_add_cancel]
[Meta.Tactic.simp.rewrite] h:1000, 1 ≤ x ==> True
[Meta.Tactic.simp.rewrite] @Nat.sub_add_cancel:1000, x - 1 + 1 ==> x
[Meta.Tactic.simp.rewrite] @eq_self:1000, (if h : 1 ≤ x then x else 0) = if _h : 1 ≤ x then x else 0 ==> True
Try this: simp only [and_self]
[Meta.Tactic.simp.rewrite] and_self:1000, b ∧ b ==> b
[Meta.Tactic.simp.rewrite] iff_self:1000, a ∧ b ↔ a ∧ b ==> True
Try this: simp only [my_thm]
[Meta.Tactic.simp.rewrite] @my_thm:1000, b ∧ b ==> b
[Meta.Tactic.simp.rewrite] @eq_self:1000, (a ∧ b) = (a ∧ b) ==> True
Try this: simp (discharger := sorry) only [Nat.sub_add_cancel]
simp_trace.lean:85:0-85:7: warning: declaration uses 'sorry'
[Meta.Tactic.simp.rewrite] @Nat.sub_add_cancel:1000, x - 1 + 1 ==> x
[Meta.Tactic.simp.rewrite] @eq_self:1000, x = x ==> True
Try this: simp only [bla, h] at *
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
Try this: simp only [bla, h] at h' 
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
Try this: simp only [bla, h, List.length_append] at *
simp_trace.lean:101:101-102:40: error: unsolved goals
x y : Nat
α : Type
xs ys : List α
h₁ : x + x = y
h₂ : List.length xs + List.length ys = y
⊢ x = length xs
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
[Meta.Tactic.simp.rewrite] @List.length_append:1000, List.length (xs ++ ys) ==> List.length xs + List.length ys
Try this: simp only [bla, h, List.length_append] at *
simp_trace.lean:105:101-106:53: error: unsolved goals
x y : Nat
α : Type
xs ys : List α
h₁ : x + x = y
h₂ : List.length xs + List.length ys = y
⊢ x = length xs
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
[Meta.Tactic.simp.rewrite] @List.length_append:1000, List.length (xs ++ ys) ==> List.length xs + List.length ys
Try this: simp only [bla, h] at *
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
[Meta.Tactic.simp.rewrite] unfold bla, bla y ==> match h y with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h y ==> Sum.inl (y, y)
Try this: simp only [bla, h] at *
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
[Meta.Tactic.simp.rewrite] unfold bla, bla x ==> match h x with
    | Sum.inl (y, z) => y + z
    | Sum.inr val => 0
[Meta.Tactic.simp.rewrite] unfold h, h x ==> Sum.inl (x, x)
Try this: simp only [HasProp.toProp]