<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_content_type(self, ct):
 """ Add a CoRE Link Format ct attribute to the resource. :param ct: the CoRE Link Format ct attribute """

        lst = self._attributes.get("ct")
        if lst is None:
            lst = []
        if isinstance(ct, str):
            ct = defines.Content_types[ct]
        lst.append(ct)
        self._attributes["ct"] = lst