English
The composition of fst with map f g equals the composition of f with fst: fst ∘ map f g = f ∘ fst.
Русский
Составление fst с map f g равно составлению f с fst: fst ∘ map f g = f ∘ fst.
LaTeX
$$$$ \\mathrm{Prod.fst} \\circ \\mathrm{Prod.map} \\; f \\; g = f \\circ \\mathrm{Prod.fst} $$$$
Lean4
theorem map_fst' (f : α → γ) (g : β → δ) : Prod.fst ∘ map f g = f ∘ Prod.fst :=
funext <| map_fst f g