intro ab hab
f : ℝ → ℝ
huc : Continuous f
hlc : Continuous f
⊢ ∀ s ∈ open_intervals, IsOpen (f ⁻¹' s)
f : ℝ → ℝ
huc : Continuous f
hlc : Continuous f
ab : Set ℝ
hab : ab ∈ open_intervals
⊢ IsOpen (f ⁻¹' ab)