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Complete this python program:
def generate_gml(G, stringizer=None):
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#Here is the Python code you requested.

    """Generate a single entry of the graph `G` in GML format.
    Parameters
    G : NetworkX graph
    The graph to be converted to GML.
    stringizer : callable, optional
    A `stringizer` which converts non-int/non-float/non-dict values into
    strings. If it cannot convert a value into a string, it should raise a
    `ValueError` to indicate that. Default value: None.
    Returns
    lines: generator of strings
    Lines of GML data. Newlines are not appended.
    Raises
    NetworkXError
    If `stringizer` cannot convert a value into a string, or the value to
    convert is not a string while `stringizer` is None.
    See Also
    literal_stringizer
    Notes
    Graph attributes named \directed\, \multigraph\, \node\ or
    \edge\, node attributes named \id\ or \label\, edge attributes
    named \source\ or \target\ (or \key\ if `G` is a multigraph)
    are ignored because these attribute names are used to encode the graph
    structure.
    GML files are stored using a 7-bit ASCII encoding with any extended
    ASCII characters (iso8859-1) appearing as HTML character entities.
    Without specifying a `stringizer`/`destringizer`, the code is capable of
    handling `int`/`float`/`str`/`dict`/`list` data as required by the GML
    specification.  For other data types, you need to explicitly supply a
    `stringizer`/`destringizer`.
    For additional documentation on the GML file format, please see the
    `GML website <http://www.infosun.fim.uni-passau.de/Graphlet/GML/gml-tr.html>`_.
    See the module docstring :mod:`networkx.readwrite.gml` for additional details.
    Examples
    >>> G = nx.Graph()
    >>> G.add_node("1")
    >>> print("\n".join(nx.generate_gml(G)))
    graph [
    node [
    id 0
    label "1"
    >>> G = nx.OrderedMultiGraph([("a", "b"), ("a", "b")])
    >>> print("\n".join(nx.generate_gml(G)))
    graph [
    multigraph 1
    node [
    id 0
    label "a"
    node [
    id 1
    label "b"
    edge [
    source 0
    target 1
    key 0
    edge [
    source 0
    target 1
    key 1
    """
    valid_keys = re.compile('^[A-Za-z][0-9A-Za-z]*$')
    def stringize(key, value, ignored_keys, indent, in_list=False):
        if (not isinstance(key, (str, unicode))):
            raise NetworkXError(('%r    is    not    a    string' % (key,)))
        if (not valid_keys.match(key)):
            raise NetworkXError(('%r    is    not    a    valid    key' % (key,)))
        if (not isinstance(key, str)):
            key = str(key)
        if (key not in ignored_keys):
            if isinstance(value, (int, long, bool)):
                if (key == 'label'):
                    (yield ((((indent + key) + '    "') + str(value)) + '"'))
                elif (value is True):
                    (yield ((indent + key) + '    1'))
                elif (value is False):
                    (yield ((indent + key) + '    0'))
                else:
                    (yield (((indent + key) + '    ') + str(value)))
            elif isinstance(value, float):
                text = repr(value).upper()
                epos = text.rfind('E')
                if ((epos != (-1)) and (text.find('.', 0, epos) == (-1))):
                    text = ((text[:epos] + '.') + text[epos:])
                if (key == 'label'):
                    (yield ((((indent + key) + '    "') + test) + '"'))
                else:
                    (yield (((indent + key) + '    ') + text))
            elif isinstance(value, dict):
                (yield ((indent + key) + '    ['))
                next_indent = (indent + '        ')
                for (key, value) in value.items():
                    for line in stringize(key, value, (), next_indent):
                        (yield line)
                (yield (indent + ']'))
            elif (isinstance(value, (list, tuple)) and (key != 'label') and value and (not in_list)):
                next_indent = (indent + '        ')
                for val in value:
                    for line in stringize(key, val, (), next_indent, True):
                        (yield line)
            else:
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