'Return a list of string reprs from a nested list of referrers.'

def format(self, tree):

    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
