function(event, newVal, oldVal)

handler(event, newValue, oldValue)

Handlers registered on "change" events will be called back as follows:

const age = compute( 33 );

age.on( "change", function( ev, newVal, oldVal ) {
    ev; //-> {type: "change", batchNum: 5}
    newVal; //-> 34
    oldVal; //-> 33
} );

age( 34 );