'Finds the n-groups of items (leaves) reachable from a cut at depth n.
:param  n: number of groups
:type   n: int'

def groups(self, n):

    if (len(self._items) > 1):
        root = _DendrogramNode(self._merge, *self._items)
    else:
        root = self._items[0]
    return root.groups(n)
