use y
case h.left
W✝ : Type
M : KripkeModel W✝
bs : List Program
a : Program
as : List Program
IH :
  ∀ (x z : W✝),
    relate M (Program.steps (as ++ bs)) x z ↔ ∃ y, relate M (Program.steps as) x y ∧ relate M (Program.steps bs) y z
x z y : W✝
x_a_y : relate M a x y
y' : W✝
y_as_ys' : relate M (Program.steps as) y y'
ys'_bs_z : relate M (Program.steps bs) y' z
⊢ ∃ y, relate M a x y ∧ relate M (Program.steps as) y y'
no goals