check box in module pool

Hi,

have 2 check box in my screen ,,c1 = a , c2 = b if i select c1 (c1 check box first) and c2(c2 check box selected after c1) both then screen field must

contain value : a b and if i select c2 then c1 then b a ,,,and if I deselect any of the check box then value related to that check box must get removed ....

C1  'X' C2 'X'

SCREEN FIELD : A B ,IF DE-SELECT C1 THEN SCREEN FIELD : B 

IF C2 'X' C1 'X'

SCREEN FIELD: BA

KINDLY HELP............................