#10 = Get_Num("Enter the key: positive to cipher, negative to de-cipher: ", STATLINE)

Goto_Pos(Block_Begin)
while(Cur_Pos < Block_End) {
    #11 = Cur_Char & 0x60 + 1
    if (Cur_Char >= 'A') {
        Ins_Char((Cur_Char - #11 + 26 + #10) % 26 + #11, OVERWRITE)
    } else {
	Char(1)
    }
}