simp only [isAlphaEqvVarList] at h1
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
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 : isAlphaEqvVar binders xs_hd ys_hd ∧ isAlphaEqvVarList binders xs_tl ys_tl
⊢ (xs_hd :: xs_tl).length = (ys_hd :: ys_tl).length