def colless(self, normalize='leaves'):
        '''Compute the Colless balance index of this ``Tree``. If the tree has polytomies, they will be randomly resolved

        Args:
            ``normalize`` (``str``): How to normalize the Colless index (if at all)

            * ``None`` to not normalize

            * ``"leaves"`` to normalize by the number of leaves

            * ``"yule"`` to normalize to the Yule model

            * ``"pda"`` to normalize to the Proportional to Distinguishable Arrangements model

        Returns:
            ``float``: Colless index (either normalized or not)
        '''
        t_res = copy(self); t_res.resolve_polytomies(); leaves_below = dict(); n = 0; I = 0
        for node in t_res.traverse_postorder():
            if node.is_leaf():
                leaves_below[node] = 1; n += 1
            else:
                cl,cr = node.children; nl = leaves_below[cl]; nr = leaves_below[cr]
                leaves_below[node] = nl+nr; I += abs(nl-nr)
        if normalize is None or normalize is False:
            return I
        elif not isinstance(normalize,str):
            raise TypeError("normalize must be None or a string")
        normalize = normalize.lower()
        if normalize == 'leaves':
            return (2.*I)/((n-1)*(n-2))
        elif normalize == 'yule':
            return (I - n*log(n) - n*(EULER_GAMMA-1-log(2)))/n
        elif normalize == 'pda':
            return I/(n**1.5)
        else:
            raise RuntimeError("normalize must be None, 'leaves', 'yule', or 'pda'")
Compute the Colless balance index of this ``Tree``. If the tree has polytomies, they will be randomly resolved

        Args:
            ``normalize`` (``str``): How to normalize the Colless index (if at all)

            * ``None`` to not normalize

            * ``"leaves"`` to normalize by the number of leaves

            * ``"yule"`` to normalize to the Yule model

            * ``"pda"`` to normalize to the Proportional to Distinguishable Arrangements model

        Returns:
            ``float``: Colless index (either normalized or not)