case cons child' subpath => simp [prefix?, h]; 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
⊢ prefix? (cons child (cons child' subpath)) = some (cons child subprefix)
no goals