human
What is a the Python code you requested given the following:
Complete the below
@not_implemented_for('directed')
def generate_sparse6(G, nodes=None, header=True):

    ''''Generate sparse6 format string from an undirected graph.
    Parameters
    G : Graph (undirected)
    nodes: list or iterable
    Nodes are labeled 0...n-1 in the order provided.  If None the ordering
    given by G.nodes() is used.
    header: bool
    If True add \'>>sparse6<<\' string to head of data
    Returns
    s : string
    String in sparse6 format
    Raises
    NetworkXError
    If the graph is directed
    Examples
    >>> G = nx.MultiGraph([(0, 1), (0, 1), (0, 1)])
    >>> nx.generate_sparse6(G)
    \'>>sparse6<<:A_\'
    See Also
    read_sparse6, parse_sparse6, write_sparse6
    Notes
    The format does not support edge or node labels.
    References
    .. [1] Sparse6 specification
    <http://users.cecs.anu.edu.au/~bdm/data/formats.html>'
    '''
assistant
#Here is the Python code you requested.
    n = G.order()
    k = 1
    while ((1 << k) < n):
        k += 1
    def enc(x):
        'Big    endian    k-bit    encoding    of    x'
        return [(1 if (x & (1 << ((k - 1) - i))) else 0) for i in range(k)]
    if (nodes is not None):
        G = G.subgraph(nodes)
    H = nx.convert_node_labels_to_integers(G, ordering='sorted')
    edges = sorted(((max(u, v), min(u, v)) for (u, v) in H.edges()))
    bits = []
    curv = 0
    for (v, u) in edges:
        if (v == curv):
            bits.append(0)
            bits.extend(enc(u))
        elif (v == (curv + 1)):
            curv += 1
            bits.append(1)
            bits.extend(enc(u))
        else:
            curv = v
            bits.append(1)
            bits.extend(enc(v))
            bits.append(0)
            bits.extend(enc(u))
    if ((k < 6) and (n == (1 << k)) and (((- len(bits)) % 6) >= k) and (curv < (n - 1))):
        bits.append(0)
        bits.extend(([1] * ((- len(bits)) % 6)))
    else:
        bits.extend(([1] * ((- len(bits)) % 6)))
    data = [((((((bits[(i + 0)] << 5) + (bits[(i + 1)] << 4)) + (bits[(i + 2)] << 3)) + (bits[(i + 3)] << 2)) + (bits[(i + 4)] << 1)) + (bits[(i + 5)] << 0)) for i in range(0, len(bits), 6)]
    res = ((':' + data_to_graph6(n_to_data(n))) + data_to_graph6(data))
    if header:
        return ('>>sparse6<<' + res)
    else:
        return res