In cases where a deconstructions would be needed to apply the same function to both sides of a pair, the overhead can be avoided by means of a property of the virtual machine intended for that purpose.
A silly definition of fan implying the following theorem is
helpful in expressing such an application.
fan]] f = ((nil,nil),((nil,f),(nil,nil)))
The virtual machine recognizes when an application has the form shown above, and uses f as a function to be applied to both sides of the argument.
fan]] f) (x,y) = (f x,f y)