form_radio and set_radio

[eluser]RickP[/eluser]
What does set_radio return?

Shouldn't form_radio('tod','all',set_radio('tod','all',TRUE)) work?

form_input('zip',set_value('zip','Enter Zip Code')) does work.

Have I misunderstood the documentation?

Have I overlooked a function?

What's the best way to do this?