How to use the weasyprint.css.utils.get_single_keyword function in weasyprint

def expand_page_break_before_after(base_url, name, tokens):
    """Expand legacy ``page-break-before`` and ``page-break-after`` properties.

    See https://www.w3.org/TR/css-break-3/#page-break-properties

    """
    keyword = get_single_keyword(tokens)
    new_name = name.split('-', 1)[1]
    if keyword in ('auto', 'left', 'right', 'avoid'):
        yield new_name, keyword
    elif keyword == 'always':
        yield new_name, 'page'

def expand_flex(base_url, name, tokens):
    """Expand the ``flex`` property."""
    keyword = get_single_keyword(tokens)
    if keyword == 'none':
        yield 'flex-grow', 0
        yield 'flex-shrink', 0
        yield 'flex-basis', 'auto'
    else:
        grow, shrink, basis = 1, 1, Dimension(0, 'px')
        grow_found, shrink_found, basis_found = False, False, False
        for token in tokens:
            # "A unitless zero that is not already preceded by two flex factors
            # must be interpreted as a flex factor."
            forced_flex_factor = (
                token.type == 'number' and token.int_value == 0 and
                not all((grow_found, shrink_found)))
            if not basis_found and not forced_flex_factor:
                new_basis = flex_basis([token])
                if new_basis is not None:

def expand_page_break_inside(base_url, name, tokens):
    """Expand the legacy ``page-break-inside`` property.

    See https://www.w3.org/TR/css-break-3/#page-break-properties

    """
    keyword = get_single_keyword(tokens)
    if keyword in ('auto', 'avoid'):
        yield 'break-inside', keyword

def generic_expander_wrapper(base_url, name, tokens):
            """Wrap the expander."""
            keyword = get_single_keyword(tokens)
            if keyword in ('inherit', 'initial'):
                results = dict.fromkeys(expanded_names, keyword)
                skip_validation = True
            else:
                skip_validation = False
                results = {}
                if wants_base_url:
                    result = wrapped(name, tokens, base_url)
                else:
                    result = wrapped(name, tokens)
                for new_name, new_token in result:
                    assert new_name in expanded_names, new_name
                    if new_name in results:
                        raise InvalidValues(
                            'got multiple %s values in a %s shorthand'
                            % (new_name.strip('-'), name))

if not required and name not in KNOWN_PROPERTIES:
        hyphens_name = name.replace('_', '-')
        if hyphens_name in KNOWN_PROPERTIES:
            raise InvalidValues('did you mean %s?' % hyphens_name)
        else:
            raise InvalidValues('unknown property')

    if not required and name not in PROPERTIES:
        raise InvalidValues('property not supported yet')

    for token in tokens:
        var_function = check_var_function(token)
        if var_function:
            return ((name, var_function),)

    keyword = get_single_keyword(tokens)
    if keyword in ('initial', 'inherit'):
        value = keyword
    else:
        function = PROPERTIES[name]
        if function.wants_base_url:
            value = function(tokens, base_url)
        else:
            value = function(tokens)
        if value is None:
            raise InvalidValues
    return ((name, value),)

def counter(tokens, default_integer):
    """``counter-increment`` and ``counter-reset`` properties validation."""
    if get_single_keyword(tokens) == 'none':
        return ()
    tokens = iter(tokens)
    token = next(tokens, None)
    assert token, 'got an empty token list'
    results = []
    while token is not None:
        if token.type != 'ident':
            return  # expected a keyword here
        counter_name = token.value
        if counter_name in ('none', 'initial', 'inherit'):
            raise InvalidValues('Invalid counter name: ' + counter_name)
        token = next(tokens, None)
        if token is not None and (
                token.type == 'number' and token.int_value is not None):
            # Found an integer. Use it and get the next token
            integer = token.int_value