checkbox in selection screen

hello!

i have a problem in selection screen with a field that

defined as checkbox.

at the initialization if a user have special parameter

i move 'X' to the checkbox field.

if i delete the 'X' from the checkbox,later on in the program i see (with the debugger) that the field still

contain 'X'.

i defined the field in the sel screen:

parameter print as checkbox.

what can i do in order to get the real values in this field?

regards

yifat