rfl
graph : Graph
start : Class graph
child : Class.Child start
subprefix : Path graph (Class.Child.class child)
child' : Class.Child (Class.Child.class child)
subpath : Path graph (Class.Child.class child')
h : prefix? (cons child' subpath) = some subprefix
⊢ (do
      let x ← some subprefix
      some (cons child x)) =
    some (cons child subprefix)
no goals