function validate(f){
    if (f.first_name.value == "") {
        alert("Please enter your first name.")
        f.first_name.focus()
        return false;
    }
    if (f.last_name.value == "") {
        alert("Please enter your last name.")
        f.last_name.focus()
        return false;
    }
    if ((f.email.value == "") && (f.tel.value == "")) {
        alert("Please enter your email address and / or your telephone number.")
        f.email.focus()
        return false;
    }
    return true;
}