other name: walk of length k
    graph theory:
    A walk (of length k) in a graph G = (V, E) is a non-empty alternating sequence `v_0e_1v_1e_1...e_(k-1)v_k` of vertices and (directed) edges in G, where `e_i = { v_i, v_(i+1) }` for all i`<`k.
    If `v_0 = v_k`, walk is called closed walk, if `v_0 != v_k`, walk is called open walk.
    If all vertices in a walk are distinct, walk has the same meaning as path.