<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_apis(self):
 """Returns set of api names referenced in this Extension :return: set of api name strings """

        out = set()
        out.update(x.api for x in self.requires if x.api)
        return out