Lean4
/-- Given `φ : NormedAddGroupHom V₁ V₂` and `ψ : NormedAddGroupHom W₁ W₂` such that
`ψ.comp f₁ = f₂.comp φ` and `ψ.comp g₁ = g₂.comp φ`, the induced morphism
`NormedAddGroupHom (f₁.equalizer g₁) (f₂.equalizer g₂)`. -/
def map (φ : NormedAddGroupHom V₁ V₂) (ψ : NormedAddGroupHom W₁ W₂) (hf : ψ.comp f₁ = f₂.comp φ)
(hg : ψ.comp g₁ = g₂.comp φ) : NormedAddGroupHom (f₁.equalizer g₁) (f₂.equalizer g₂) :=
lift (φ.comp <| ι _ _) <| by
simp only [← comp_assoc, ← hf, ← hg]
simp only [comp_assoc, comp_ι_eq f₁ g₁]