There is a need to be able to double bind 2 properties together based on a difference between them. For instance I wish to be able to have a property that represents the middle of a node in relation to the nodes parent, and when I adjust that value its associated layout value changes and visa versa. Additionaly when I adjust the difference either the layout or the middle property are modified, dependant on which of those values is not already bound to another value. (If neither than it suggested that the right most argument on setup is modified, in this case)

I have come across this exact issue when generating syntax diagrams, as most nodes are associated with each other not on the top left but an arbitrary location on the left and right hand sides.

If we take L to be layout, N to be our new property and D to be the difference.
L = N + D
N = L - D
D = L - N

It may be viewed that a difference by a constan