a few more questions ?

hi all,
i have some more questions to ask :
the default start url is this : > start_url:        http://www.htdig.org
my questions :
1. if i want to change this start url, do i have to purge the database first
? if yes, how can i purge the existing database ? [with a command or
something else ?]
2. am i able to index more than one url ? i mean i want to create search
index for xxx.com, yyy.com, zzz.com.
if its possible then how i can insert the new urls in htdig.conf ?
should i add at the end of default site and separate them with commas :
start_url xxx.com, yyy.com, zzz.com
or should i add like this :
start_url xxx.com
start_url yyy.com
start_url zzz.com ?
3. what exactly does htdig search ? what is the main criteria ? it just
returns the LINKS that contains the word i look for ?
many thanks in advance.

Cheers,
MFA