cases path₁
case mpr
graph : Graph
start : Class graph
child : Class.Child start
path₁ path₂ : Path graph (Class.Child.class child)
h : prefix? (cons child path₁) = some (cons child path₂)
⊢ prefix? path₁ = some path₂
case mpr.nil
graph : Graph
start : Class graph
child : Class.Child start
path₂ : Path graph (Class.Child.class child)
h : prefix? (cons child nil) = some (cons child path₂)
⊢ prefix? nil = some path₂

case mpr.cons
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 (cons child✝ a✝)) = some (cons child path₂)
⊢ prefix? (cons child✝ a✝) = some path₂