cases ys
binders : List (VarName × VarName)
ys : List VarName
h1 : isAlphaEqvVarList binders [] ys
⊢ [].length = ys.length
case nil
binders : List (VarName × VarName)
h1 : isAlphaEqvVarList binders [] []
⊢ [].length = [].length

case cons
binders : List (VarName × VarName)
head✝ : VarName
tail✝ : List VarName
h1 : isAlphaEqvVarList binders [] (head✝ :: tail✝)
⊢ [].length = (head✝ :: tail✝).length