function on(key) {

    const down = listener => add(listener, keydown);
    const up = listener => add(listener, keyup);

    function add(listener, filters) {
        get(key, filters).listeners.
        add(listener);
        
        return { down, up };
    };  return { down, up };
}
// listener and ADDING them to the Data Structure of listeners. 
listener and ADDING them to the Data Structure of listeners.
listener and ADDING them to the Data Structure of listeners .
function on(key) {
    const down = listener => add(listener, keydown);
    const up = listener => add(listener, keyup);
    function add(listener, filters) {
        get(key, filters).listeners.
        add(listener);
        return { down, up };
    };  return { down, up };
}
function on ( key ) { const down = listener => add ( listener , keydown ) ; const up = listener => add ( listener , keyup ) ; function add ( listener , filters ) { get ( key , filters ) . listeners . add ( listener ) ; return { down , up } ; } ; return { down , up } ; }
listener and ADDING them to the Data Structure of listeners.
listener and ADDING them to the Data Structure of listeners .