fn allocate_if_new<'s, 'b>( v: Vertex<'s, 'b>, alloc: &'b Bump, seen: &mut HashMap<&Vertex<'s, 'b>, SmallVec<[&'b Vertex<'s, 'b>; 2]>, BuildHasherDefault<FxHasher>>) -> &'b Vertex<'s, 'b>