<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def unique_authors(self, limit):
 """Unique list of authors, but preserving order."""

        seen = set()
        if limit == 0:
            limit = None
        seen_add = seen.add  # Assign to variable, so not resolved each time
        return [x.author for x in self.sorted_commits[:limit]
                if not (x.author in seen or seen_add(x.author))]