There Can Be Only One...

Highlander.js

function there_can_be_only_one_highlander(the_one) {
     var state_speed = 500,
     immortals = ['#welcome-box', '#guest-box', '#login-box', '#register-box'],
     to_kill = [],
     key = null;

     for (key in immortals) {
     	if (immortals[key] !== the_one) {
     		if (typeof immortals[key] === 'string') {
     			to_kill.push(immortals[key]);
     		}
     	}
     }

     jQuery(to_kill.join(',')).fadeOut(state_speed);

     setTimeout(function() {jQuery(the_one).slideDown(state_speed);}, state_speed);
}