Re-populating forms

[eluser]Perkin5[/eluser]
I have set up a form using the form-helper syntax and the input looks like this:

Code:

echo form_input('username','','id="username"');

and I want to repopulate it with POST array data. I'm assuming you have to use set_value(). The user guide shows you how to do it when you're not using the form helper but I can't find how to do it with the form helper.

I've tried this:

Code:

echo form_input('username','','id="username",value="set_value('username')"');

but it doesn't work, and neither does this:

Code:

echo form_input('username','set_value('username')','id="username"');

Can anyone put me wise?