def METHOD_NAME(self, tree):
    """Return a list of string reprs from a nested list of referrers."""
    output = []
    def ascend(branch, depth=1):
        for parent, grandparents in branch:
            output.append(('    ' * depth) + self._format(parent))
            if grandparents:
                ascend(grandparents, depth + 1)
    ascend(tree)
    return output