In this section:
The SETF call-back takes two arguments each of double precision type. When the call-back is called, the first argument is set to the value of the second at the time when winio@ is called. SETF returns the value 1 so that any necessary updates take place. Here is an example.
DOUBLE PRECISON v,x v=0d0
The variable v is initially zero and takes the value 1 (rather than 2) when the button is clicked. The SET callback performs the same operation on integers.