function isset(variable_name) {
    variable_name = document.getElementById(variable_name).value;
    try {
	if (typeof(eval(variable_name)) != 'undefined')
	if (eval(variable_name) != null)
	    return true;
    } catch(e) { }
    return false;
    }

//validar correo electronico
function isEmailAddress( id_elemento ){
    var s = document.getElementById(id_elemento).value;
    var filter=/^[A-Za-z][A-Za-z0-9_]*@[A-Za-z0-9_]+\.[A-Za-z0-9_.]+[A-za-z]$/;
    if (filter.test(s))
        return true;
    else
        return false;
	}
	
var string;
function empty( string ){
        var campo = document.getElementById( string ).value;
        if ( campo.length == 0 )
            return true;
        else
            return false;
        }
	
var nav4 = window.Event ? true : false;
function acceptNum(evt){	
	var key = nav4 ? evt.which : evt.keyCode;	
	return (key <= 13 || (key >= 48 && key <= 57));
	}