def series(expr, x, x0, n, removeO=False):
    """
    sympy bugs avoided
    """
    # expr_series = expr.series(x, x0, n)
    # return expr_series.removeO() if removeO else expr_series
    expansion = list()
    for t in expr.lseries(x, x0):
        p = t.as_coeff_exponent(x)[1]
        if p < n:
            expansion.append(t)
        else:
            break
    if not removeO:
        expansion.append(sympy.O(x**n))
    return sympy.Add(*expansion)