<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def purge_unused(self, pass_count=3):
 """Append an purge model entry to the journal. This instructs Revit to purge the open model. Args: pass_count (int):
 number of times to execute the purge. default is 3 """

        for purge_count in range(0, pass_count):
            self._add_entry(templates.PROJECT_PURGE)