-- Mathlib/Analysis/ODE/Gronwall.lean
theorem dist_le_of_approx_trajectories_ODE_of_mem
    (hv : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t))
    (hf : ContinuousOn f (Icc a b))
    (hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t)
    (f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf)
    (hfs : ∀ t ∈ Ico a b, f t ∈ s t)
    (hg : ContinuousOn g (Icc a b))
    (hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t)
    (g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg)
    (hgs : ∀ t ∈ Ico a b, g t ∈ s t)
    (ha : dist (f a) (g a) ≤ δ) :
    ∀ t ∈ Icc a b, dist (f t) (g t) ≤ gronwallBound δ K (εf + εg) (t - a)

Proof state:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv✝ : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t
t : ℝ
ht : t ∈ Ico a b
this : dist (f' t) (g' t) ≤ dist (f' t) (v t (f t)) + dist (g' t) (v t (g t)) + dist (v t (f t)) (v t (g t))
hv : dist (v t (f t)) (v t (g t)) ≤ ↑K * dist (f t) (g t)
⊢ dist (f' t) (g' t) ≤ ↑K * dist (f t) (g t) + (εf + εg)

Tactic:
refine this.trans ((add_le_add (add_le_add (f_bound t ht) (g_bound t ht)) hv).trans ?_)

Result:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv✝ : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t
t : ℝ
ht : t ∈ Ico a b
this : dist (f' t) (g' t) ≤ dist (f' t) (v t (f t)) + dist (g' t) (v t (g t)) + dist (v t (f t)) (v t (g t))
hv : dist (v t (f t)) (v t (g t)) ≤ ↑K * dist (f t) (g t)
⊢ εf + εg + ↑K * dist (f t) (g t) ≤ ↑K * dist (f t) (g t) + (εf + εg)

Proof state:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv✝ : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t
t : ℝ
ht : t ∈ Ico a b
this : dist (f' t) (g' t) ≤ dist (f' t) (v t (f t)) + dist (g' t) (v t (g t)) + dist (v t (f t)) (v t (g t))
hv : dist (v t (f t)) (v t (g t)) ≤ ↑K * dist (f t) (g t)
⊢ ‖f' t - g' t‖ ≤ ↑K * ‖f t - g t‖ + (εf + εg)

Tactic:
rw [← dist_eq_norm, ← dist_eq_norm]

Result:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv✝ : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t
t : ℝ
ht : t ∈ Ico a b
this : dist (f' t) (g' t) ≤ dist (f' t) (v t (f t)) + dist (g' t) (v t (g t)) + dist (v t (f t)) (v t (g t))
hv : dist (v t (f t)) (v t (g t)) ≤ ↑K * dist (f t) (g t)
⊢ dist (f' t) (g' t) ≤ ↑K * dist (f t) (g t) + (εf + εg)

Proof state:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : dist (f a) (g a) ≤ δ
⊢ ∀ t ∈ Icc a b, dist (f t) (g t) ≤ gronwallBound δ (↑K) (εf + εg) (t - a)

Tactic:
simp only [dist_eq_norm] at ha ⊢

Result:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
⊢ ∀ t ∈ Icc a b, ‖f t - g t‖ ≤ gronwallBound δ (↑K) (εf + εg) (t - a)

Proof state:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
⊢ ∀ t ∈ Icc a b, ‖f t - g t‖ ≤ gronwallBound δ (↑K) (εf + εg) (t - a)

Tactic:
have h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t :=
  fun t ht => (hf' t ht).sub (hg' t ht)

Result:
E : Type u_1
inst✝¹ : NormedAddCommGroup E
inst✝ : NormedSpace ℝ E
v : ℝ → E → E
s : ℝ → Set E
K : ℝ≥0
f g f' g' : ℝ → E
a b εf εg δ : ℝ
hv : ∀ t ∈ Ico a b, LipschitzOnWith K (v t) (s t)
hf : ContinuousOn f (Icc a b)
hf' : ∀ t ∈ Ico a b, HasDerivWithinAt f (f' t) (Ici t) t
f_bound : ∀ t ∈ Ico a b, dist (f' t) (v t (f t)) ≤ εf
hfs : ∀ t ∈ Ico a b, f t ∈ s t
hg : ContinuousOn g (Icc a b)
hg' : ∀ t ∈ Ico a b, HasDerivWithinAt g (g' t) (Ici t) t
g_bound : ∀ t ∈ Ico a b, dist (g' t) (v t (g t)) ≤ εg
hgs : ∀ t ∈ Ico a b, g t ∈ s t
ha : ‖f a - g a‖ ≤ δ
h_deriv : ∀ t ∈ Ico a b, HasDerivWithinAt (fun t => f t - g t) (f' t - g' t) (Ici t) t
⊢ ∀ t ∈ Icc a b, ‖f t - g t‖ ≤ gronwallBound δ (↑K) (εf + εg) (t - a)