Iff the value of an attribute member of the image graphobject is unset or
is a variable, fill up the value of such attribute member with the value
(it should be a constant value) of the same attribute member of the
original graphobject.
If the value of an attribute member of the original graphobject is unset
or is a variable, fill up such attribute value with the value (it should
be a constant value) of the same attribute member of the image
graphobject.
This method should be overridden by classes that wish to customize or
filter the actual event depending on the respective observer and/or its
own framework (index transformation, id change).
A special value designating that the assignment was done in the parent
context and this context's references don't count; such a value is
permanent, it may not be changed by this value tuple.