<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def do_back(self, line):
        "Go back one step. From entity => ontology; from ontology => ontospy top level."
        if self.currentEntity:
            self.currentEntity = None
            self.prompt = _get_prompt(self.current['file'])
        else:
            self.current = None
            self.prompt = _get_prompt()