<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def submit_string(self):
 """Return a string specifying the files to submit for this project."""

        required = []
        optional = []
        for file_verifier in self.file_verifiers:
            if file_verifier.optional:
                optional.append('[{0}]'.format(file_verifier.filename))
            else:
                required.append(file_verifier.filename)
        return ' '.join(sorted(required) + sorted(optional))