walk on given graphs, beginning on start.
    Yield all found nodes, including start.

    All graph are understood as a single one,
    with merged keys and values.

def walk(start:list, graphs:iter) -> iter:
    """walk on given graphs, beginning on start.
    Yield all found nodes, including start.

    All graph are understood as a single one,
    with merged keys and values.

    """
    walked = set([start])
    stack = [start]
    while len(stack) > 0:
        *stack, curr = stack
        yield curr
        succs = it.chain.from_iterable(graph.get(curr, ()) for graph in graphs)
        for succ in succs:
            if succ not in walked:
                walked.add(curr)
                stack.append(succ)