intro h
graph✝ : Graph
a✝¹ : Class graph✝
child✝ : Class.Child a✝¹
a✝ : Path graph✝ (Class.Child.class child✝)
⊢ Path.nil ≂ Path.cons child✝ a✝ → prefix? Path.nil = prefix? (Path.cons child✝ a✝)
graph✝ : Graph
a✝¹ : Class graph✝
child✝ : Class.Child a✝¹
a✝ : Path graph✝ (Class.Child.class child✝)
h : Path.nil ≂ Path.cons child✝ a✝
⊢ prefix? Path.nil = prefix? (Path.cons child✝ a✝)