function change_class(im, id){
	document.getElementById(id).className = 'alertes-' + id + '-' + im + '';
}

function check_form(){

	if (document.getElementById('nom').value == ""){
		alert( "Please enter your last name." );
		return false ;
	}

	if (document.getElementById('pre').value == ""){
		alert( "Please enter your first name." );
		return false ;
	}

	if (document.getElementById('mel').value == ""){
		alert( "Please enter your email address." );
		return false ;
	}

	return true ;
}