add calculating methods to bind

[Tom Eugelink]

I would like to add a snap method to a calculating bind. For example:

startXProperty().bind( heightProperty().multiply(0.2).snap() )

One option would be to extend the DoubleProperty that is used for the 
heightProperty, but that would only work for situations where I am the owner of 
the source. If the binding were reversed (I don't own the source for Line) that 
will not work.

The other option would be to create a special bind method:

bindSnap( startXProperty(), heightProperty().multiply(0.2) )

But that would mean duplicating the code in the bind method, and I do not want 
that (if it is possible at all). And it is not possible to register a change 
listener to startXProperty and snap the result, because there is none, the 
value is calculated on the fly. So I really need a way to get into the chain.

Any suggestions?

Tom