What is _u function

hi every one
in the following line of code
What is _u Function ?

**header("Location: " . _u('index.php?app=main&inc=core_auth&route=login'));**

Its a function to modify URL. But, currently nothing modified, so basically its just echo-ing the given parameter.

anton

ok
Thanks for reply

Where is it Defined, In Which File

here:

and themes_url here:

anton

got it …
thanks …