ABCD
!2!!0!
After unset
BCD
BCD
!2!A!0!
After unset
CD
After unset
C
C
!2!C!0!
After unset

After unset
C
After unset
BD
After unset
BD
BD
!2!C!0!
After unset
D
After unset
D
D
!2!D!0!
After unset

After unset
BD
AB
CC
CD
AB
CB
DD
CD