'Returns the rank of the Prufer sequence.
Examples
>>> from sympy.combinatorics.prufer import Prufer
>>> p = Prufer([[0, 3], [1, 3], [2, 3], [3, 4], [4, 5]])
>>> p.rank
778
>>> p.next(1).rank
779
>>> p.prev().rank
777
See Also
prufer_rank, next, prev, size'

@property
def rank(self):

    if (self._rank is None):
        self._rank = self.prufer_rank()
    return self._rank
