function activate( instances ) {
  const instancesArray = [].concat( instances );

  // if no components were found as ancestors of the event target,
  // effectively deactivate keydown handling by capping the set of instances
  // with `null`.
  if ( !instancesArray.length ) {
    _instances.add( null );
  } else {
    _instances.delete( null );

    // deleting and then adding the instance(s) has the effect of sorting the set
    // according to instance activation (ascending)
    instancesArray.forEach( instance => {
      _instances.delete( instance );
      _instances.add( instance );
    });
  }
}
activate

@access public
@param {object} instance Instantiated class that extended React.Component, to be focused to receive keydown events