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
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.