code: def available_ports(self):
          
          from mbuild.port import Port
          return [port for port in self.labels.values()
                  if isinstance(port, Port) and not port.used]
docs: Return all unoccupied Ports referenced by this Compound.
  
          Returns
          -------
          list of mb.Compound
              A list of all unoccupied ports referenced by the Compound