COPYFROM 23
COPYTO   20
COPYTO   21

first:
INBOX
JUMPZ    second
COPYTO   [23]
BUMPUP   23
JUMP     first

second:
COPYTO   [23]

input:
INBOX
JUMPZ    replace0
COPYTO   19
COPYFROM [20]
JUMPZ    out
SUB      19
JUMPN    out
COPYFROM 19
SUB      [20]
JUMPN    replace
BUMPUP   20
JUMP     input

replace:
COPYFROM 19
COPYTO   [20]

continue:
BUMPUP   20
INBOX
JUMPZ    out2
COPYTO   [20]
JUMP     continue


replace0:
COPYTO   [20]
JUMP     out

out2:
COPYTO   [20]

out:
COPYFROM [21]
JUMPZ    end
OUTBOX
BUMPUP   21
JUMP     out

end: