human
Solve the following python programming problem given the following code:
def _pack_instance_onto_cores(available_siblings, instance_cell, host_cell_id, threads_per_core = 1) : 
    "Pack an instance onto a set of siblings

    :param available_siblings: list of sets of CPU id's - available
                               siblings per core
    :param instance_cell: An instance of objects.InstanceNUMACell describing
                          the pinning requirements of the instance
    :param threads_per_core: number of threads per core in host's cell

    :returns: An instance of objects.InstanceNUMACell containing the pinning
              information, and potentially a new topology to be exposed to the
              instance. None if there is no valid way to satisfy the sibling
              requirements for the instance.

    This method will calculate the pinning for the given instance and it's
    topology, making sure that hyperthreads of the instance match up with
    those of the host when the pinning takes effect.

    Currently the strategy for packing is to prefer siblings and try use cores
    evenly, by using emptier cores first. This is achieved by the way we order
    cores in the sibling_sets structure, and the order in which we iterate
    through it.

    The main packing loop that iterates over the sibling_sets dictionary will
    not currently try to look for a fit that maximizes number of siblings, but
    will simply rely on the iteration ordering and picking the first viable
    placement.
    " 
    sibling_sets = collections.defaultdict (list) 
    for sib in available_siblings : 
        for threads_no in range (1, (len (sib) + 1)) : 
            sibling_sets [threads_no].append (sib) 
pinning = None 
    threads_no = 1 
    def _orphans(instance_cell, threads_per_core) : 
        'Number of instance CPUs which will not fill up a host core.

        Best explained by an example: consider set of free host cores as such:
            [(0, 1), (3, 5), (6, 7, 8)]
        This would be a case of 2 threads_per_core AKA an entry for 2 in the
        sibling_sets...

    
    

    Fix the buggy line: if (sibling_sets <= 1) :
assistant
#Here is Python program to fix the buggy line: if (sibling_sets <= 1) :
.

if (threads_no <= 1) :