case cons ys_hd ys_tl =>
  simp only [isAlphaEqvVarList] at h1
  simp
  cases h1
  case intro h1_left h1_right =>
    exact xs_ih ys_tl h1_right
binders : List (VarName × VarName)
xs_hd : VarName
xs_tl : List VarName
xs_ih : ∀ (ys : List VarName), isAlphaEqvVarList binders xs_tl ys → xs_tl.length = ys.length
ys_hd : VarName
ys_tl : List VarName
h1 : isAlphaEqvVarList binders (xs_hd :: xs_tl) (ys_hd :: ys_tl)
⊢ (xs_hd :: xs_tl).length = (ys_hd :: ys_tl).length
no goals