function insert(num) {
document.form.textview.value = document.form.textview.value + num;
}
function clean() {
document.form.textview.value = "";
}
function equal() {
var exp = document.form.textview.value;
if (exp) {
document.form.textview.value = eval(exp)
}
}
function back() {
var exp = document.form.textview.value;
document.form.textview.value = exp.substring(0, exp.length - 1);
}