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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_reserved_bindings(vlan_id, instance_id, switch_ip=None, port_id=None):
 """Lists reserved bindings."""

    LOG.debug("get_reserved_bindings() called")
    if port_id:
        return _lookup_all_nexus_bindings(vlan_id=vlan_id,
                                          switch_ip=switch_ip,
                                          instance_id=instance_id,
                                          port_id=port_id)
    elif switch_ip:
        return _lookup_all_nexus_bindings(vlan_id=vlan_id,
                                          switch_ip=switch_ip,
                                          instance_id=instance_id)
    else:
        return _lookup_all_nexus_bindings(vlan_id=vlan_id,
                                          instance_id=instance_id)