Forums Scripts & Popups Manipulating variables?

I have a code like..

Input
!a

Output
message store1
message store2
message store3


How do I make it so that I can make it so I can "activate" certain parts of the code so it'll be something like this..

Input
!a

Output
message store1
message store2
message store3

And if I don't have message 3 to be displayed, I can do...

Input
"some activation code"
!a

Output
message store1
message store2

I know I can change the variable to something like !b and have it output store1 and store2, but I would like to be able to choose which messages to be displayed from the !a variable only. Is this sort of thing possible? Thanks..