Try doing some basic maths questions in the Lean Theorem Prover. Functions, real numbers, equivalence relations and groups. Click on README.md and then on "Open in CoCalc with one click".
License: APACHE
import tactic open function theorem challenge4 (X Y Z : Type) (f : X → Y) (g : Y → Z) : surjective (g ∘ f) → surjective g := begin sorry end