-- Mathlib/Analysis/Calculus/Implicit.lean
theorem to_implicitFunctionOfComplemented (hf : HasStrictFDerivAt f f' a) (hf' : range f' = ⊤)
    (hker : (ker f').ClosedComplemented) :
    HasStrictFDerivAt (hf.implicitFunctionOfComplemented f f' hf' hker (f a))
      (ker f').subtypeL 0

Proof state:
case h.e'_13
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ 0 =
    (implicitFunctionDataOfComplemented f f' hf hf' hker).rightFun
      (implicitFunctionDataOfComplemented f f' hf hf' hker).pt

case convert_1
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ (implicitFunctionDataOfComplemented f f' hf hf' hker).rightDeriv.comp (LinearMap.ker f').subtypeL =
    ContinuousLinearMap.id 𝕜 ↥(LinearMap.ker f')

case convert_2
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ (implicitFunctionDataOfComplemented f f' hf hf' hker).leftDeriv.comp (LinearMap.ker f').subtypeL = 0

Tactic:
swap

Result:
case convert_1
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ (implicitFunctionDataOfComplemented f f' hf hf' hker).rightDeriv.comp (LinearMap.ker f').subtypeL =
    ContinuousLinearMap.id 𝕜 ↥(LinearMap.ker f')

case h.e'_13
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ 0 =
    (implicitFunctionDataOfComplemented f f' hf hf' hker).rightFun
      (implicitFunctionDataOfComplemented f f' hf hf' hker).pt

case convert_2
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ (implicitFunctionDataOfComplemented f f' hf hf' hker).leftDeriv.comp (LinearMap.ker f').subtypeL = 0

Proof state:
case convert_2
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ (implicitFunctionDataOfComplemented f f' hf hf' hker).leftDeriv.comp (LinearMap.ker f').subtypeL = 0

Tactic:
ext

Result:
case convert_2.h
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
x✝ : ↥(LinearMap.ker f')
⊢ ((implicitFunctionDataOfComplemented f f' hf hf' hker).leftDeriv.comp (LinearMap.ker f').subtypeL) x✝ = 0 x✝

Proof state:
case h.e'_13
𝕜 : Type u_1
inst✝⁶ : NontriviallyNormedField 𝕜
E : Type u_2
inst✝⁵ : NormedAddCommGroup E
inst✝⁴ : NormedSpace 𝕜 E
inst✝³ : CompleteSpace E
F : Type u_3
inst✝² : NormedAddCommGroup F
inst✝¹ : NormedSpace 𝕜 F
inst✝ : CompleteSpace F
f : E → F
f' : E →L[𝕜] F
a : E
hf : HasStrictFDerivAt f f' a
hf' : range f' = ⊤
hker : (LinearMap.ker f').ClosedComplemented
⊢ 0 =
    (implicitFunctionDataOfComplemented f f' hf hf' hker).rightFun
      (implicitFunctionDataOfComplemented f f' hf hf' hker).pt

Tactic:
simp only [implicitFunctionDataOfComplemented, map_sub, sub_self]

Result:
no goals