human
I wrote the below code

    has_equality = False
    def arg_expand(bool_expr):
        '\n                                Recursively    expands    the    arguments    of    an    Boolean    Function\n                                '
        for arg in bool_expr.args:
            if isinstance(arg, BooleanFunction):
                arg_expand(arg)
            elif isinstance(arg, Relational):
                arg_list.append(arg)
    arg_list = []
    if isinstance(expr, BooleanFunction):
        arg_expand(expr)
        if any((isinstance(e, (Equality, GreaterThan, LessThan)) for e in arg_list)):
            has_equality = True
    elif (not isinstance(expr, Relational)):
        expr = Eq(expr, 0)
        has_equality = True
    elif isinstance(expr, (Equality, GreaterThan, LessThan)):
        has_equality = True
    xyvar = [i for i in (x_var, y_var) if (i is not None)]
    free_symbols = expr.free_symbols
    range_symbols = Tuple(*flatten(xyvar)).free_symbols
    undeclared = (free_symbols - range_symbols)
    if (len((free_symbols & range_symbols)) > 2):
        raise NotImplementedError('Implicit    plotting    is    not    implemented    for    more    than    2    variables')
    default_range = Tuple((-5), 5)
    def _range_tuple(s):
        if isinstance(s, Symbol):
            return (Tuple(s) + default_range)
        if (len(s) == 3):
            return Tuple(*s)
        raise ValueError(('symbol    or    `(symbol,    min,    max)`    expected    but    got    %s' % s))
    if (len(xyvar) == 0):
        xyvar = list(_sort_gens(free_symbols))
    var_start_end_x = _range_tuple(xyvar[0])
    x = var_start_end_x[0]
    if (len(xyvar) != 2):
        if ((x in undeclared) or (not undeclared)):
            xyvar.append(Dummy(('f(%s)' % x.name)))
        else:
            xyvar.append(undeclared.pop())
    var_start_end_y = _range_tuple(xyvar[1])
    use_interval = kwargs.pop('adaptive', True)
    nb_of_points = kwargs.pop('points', 300)
    depth = kwargs.pop('depth', 0)
    line_color = kwargs.pop('line_color', 'blue')
   ...

What's a good function header?
assistant
#Here's a good function header.

@doctest_depends_on(modules=('matplotlib',))
def plot_implicit(expr, x_var=None, y_var=None, **kwargs):