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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 available_ports(self):
 """Return all unoccupied Ports referenced by this Compound. Returns ------- list of mb.Compound A list of all unoccupied ports referenced by the Compound """

        from mbuild.port import Port
        return [port for port in self.labels.values()
                if isinstance(port, Port) and not port.used]