The SET call-back takes two arguments each of type INTEGER. When the call-back
is called, the first argument is set to the value of the second at the time
when winio@ is called. SET returns the value 1 so that any necessary updates
take place. Here is an example.
The variable v is initially zero and tekes the value 1 (rather than 2)
when the button is clicked. The SETF callback performs the same operation on floating-point numbers.