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Changes between Initial Version and Version 1 of Ticket #14711, comment 121

initial	v1
48	48	I guess one could test `self._parent is None`, rather than typetest stuff. This should actually be faster.
49	49
50		Concerning "reasons": The parent is unset, because otherwise we have a chain of ~~from~~ references from the map to the domain, namely via the parent (i.e., the homset). Hence, having a weak reference from the map to the domain would be futile if there is a reference from the map to its parent. Note that alternatively one could have a weak reference from the ''homset'' to the domain. But I think we have agreed above that we don't want this as a default.
	50	Concerning "reasons": The parent is unset, because otherwise we have a chain of references from the map to the domain, namely via the parent (i.e., the homset). Hence, having a weak reference from the map to the domain would be futile if there is a reference from the map to its parent. Note that alternatively one could have a weak reference from the ''homset'' to the domain. But I think we have agreed above that we don't want this as a default.