how to mask some packages when emerge -e world

[double_crane]

3Q for reading my problem;

I've used Gentoo as desktop over half an year. when I feel the system is not so quick or satble , I'd like to emerge -e world .

I know it'll re-emerge packages in /var/lib/portage/world ,

here is my question , if I don't want it to re-emerge libreoffice or firefox ... say , mask these package keep it as used to be ,(because it just wastes a lot of time)

how how can I mask them so as to not be changed by "emerge -e world" ,to decline my compile time?

can I just add # before these lines in /var/lib/portage/world ? or to delete these lines in /var/lib/portage/world ?

if I delete the lines or add # , will they be unmerged ? I've never tried it before

thankyou

[kurly]

From the man page:

[double_crane]

thank you

what you've said really works
forgive my lazy not to read the help document because of english.

thank you again