'Merges nodes at given indices in the dendrogram. The nodes will be
combined which then replaces the first node specified. All other nodes
involved in the merge will be removed.
:param  indices: indices of the items to merge (at least two)
:type   indices: seq of int'

def merge(self, *indices):

    assert (len(indices) >= 2)
    node = _DendrogramNode(self._merge, *[self._items[i] for i in indices])
    self._merge += 1
    self._items[indices[0]] = node
    for i in indices[1:]:
        del self._items[i]
