def miss_defaulting(self) -> "PerThreeMachineDefaultable[T.Optional[_T]]":
        """Unset definition duplicated from their previous to None

        This is the inverse of ''default_missing''. By removing defaulted
        machines, we can elaborate the original and then redefault them and thus
        avoid repeating the elaboration explicitly.
        """
        unfreeze: PerThreeMachineDefaultable[T.Optional[_T]] = PerThreeMachineDefaultable()
        unfreeze.build = self.build
        unfreeze.host = self.host
        unfreeze.target = self.target
        if unfreeze.target == unfreeze.host:
            unfreeze.target = None
        if unfreeze.host == unfreeze.build:
            unfreeze.host = None
        return unfreeze
Unset definition duplicated from their previous to None

        This is the inverse of ''default_missing''. By removing defaulted
        machines, we can elaborate the original and then redefault them and thus
        avoid repeating the elaboration explicitly.
        
mesonbuild/utils/universal.py