<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def check_is_a_string(var, allow_none=False):
 """ Calls is_a_string and raises a type error if the check fails. """

    if not is_a_string(var, allow_none=allow_none):
        raise TypeError("var must be a string, however type(var) is {}"
                        .format(type(var)))