case nil => simp [prefix?, Option.isSome]
graph : Graph
start start✝ : Class graph
child₁ : Class.Child start✝
hi : isCons nil = true → Option.isSome (prefix? nil) = true
h : isCons (cons child₁ nil) = true
⊢ Option.isSome (prefix? (cons child₁ nil)) = true
no goals