apply Set.eq_of_subset_of_subset
Node : Type
inst✝ : DecidableEq Node
g : Graph Node
xs ys : List Node
⊢ reachable_from_list g (xs ++ ys) = reachable_from_list g xs ∪ reachable_from_list g ys
case a
Node : Type
inst✝ : DecidableEq Node
g : Graph Node
xs ys : List Node
⊢ reachable_from_list g (xs ++ ys) ⊆ reachable_from_list g xs ∪ reachable_from_list g ys

case a
Node : Type
inst✝ : DecidableEq Node
g : Graph Node
xs ys : List Node
⊢ reachable_from_list g xs ∪ reachable_from_list g ys ⊆ reachable_from_list g (xs ++ ys)