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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 external_name(self):
 """Returns the modulename.executable string that uniquely identifies the executable that this dependency points to."""

        target = self.target
        if target is not None:
            return "{}.{}".format(target.name.lower(), self.name)
        else:
            return "{}.{}".format(self.module.name.lower(), self.name)