∀ {X : Type u_1} [inst : TopologicalSpace X] {a₁ a₂ b₁ b₂ c₁ c₂ : X} (γ : Path a₂ b₂) (γ' : Path b₂ c₂) (ha : a₁ = a₂)
  (hb : b₁ = b₂) (hc : c₁ = c₂), (γ.cast ha hb).trans (γ'.cast hb hc) = (γ.trans γ').cast ha hc