simp [prefix?, h]
graph : Graph
start : Class graph
child : Class.Child start
path₂ : Path graph (Class.Child.class child)
child✝ : Class.Child (Class.Child.class child)
a✝ : Path graph (Class.Child.class child✝)
h : prefix? (cons child✝ a✝) = some path₂
⊢ prefix? (cons child (cons child✝ a✝)) = some (cons child path₂)
graph : Graph
start : Class graph
child : Class.Child start
path₂ : Path graph (Class.Child.class child)
child✝ : Class.Child (Class.Child.class child)
a✝ : Path graph (Class.Child.class child✝)
h : prefix? (cons child✝ a✝) = some path₂
⊢ (do
      let x ← some path₂
      some (cons child x)) =
    some (cons child path₂)