Function is_neighbor 

pub fn is_neighbor<V>(
    q: &GraphQuery<V>,
    weight: &TraversalWeight<V>,
    a: &Pattern<V>,
    b: &Pattern<V>,
) -> bool
where
    V: GraphValue + Clone,
    V::Id: Clone + Eq + Hash,

Returns true if b is directly reachable from a in one hop under weight.