How to replace format-like codes in a string?

[Marcin Borkowski]

Hi list,

I want to write a function like `format', which gets a string with some
codes embedded (like "%a", "%b" etc., and "%%" to get a percentage
sign), and convert them to something (say, "%a" to the value of var-a,
"%b" to var-b etc.).  Is there any facility in Elisp to help with that
or do I have to do a search-and-replace manually?

Best,

-- 
Marcin Borkowski