code    ChIn=7, ChOut=8;
int     C, CC;
repeat  C:= ChIn(1);  CC:= C&~$20;  \CC handles lowercase too
        ChOut(0, C + (if CC>=^A & CC<=^M then +13
                 else if CC>=^N & CC<=^Z then -13
                 else 0));
until   C = $1A;  \EOF