∀ {E : Type u_1} [inst : NormedAddCommGroup E] [inst_1 : NormedSpace ℝ E] {H : Type u_2} [inst_2 : TopologicalSpace H]
  {I : ModelWithCorners ℝ E H} {M : Type u_3} [inst_3 : TopologicalSpace M] [inst_4 : ChartedSpace H M] {x y : ℝ}
  [h : Fact (x < y)] {f : ↑(Set.Icc x y) → M} {w : ↑(Set.Icc x y)},
  MDiffAt[Set.Icc x y] (f ∘ Set.projIcc x y ⋯) ↑w ↔ MDiffAt f w