Nybbleist

There is a list of nybbles acting as both a stack and queue at the same time, and 2 variables X and Y that can store 1 nybble each.

Values

n = a nybble 0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F or a variable X,Y. If you put multiple nybbles after a * or ! command, the command is executed once for each nybble in order.
v = a variable X or Y.
l = a label name consisting of one or more nybbles (some can be X or Y).
d = a (definition) label name consisting of one or more nybbles, but no X or Y.

*n Push/enqueue value n onto/in the stack/queue.
<v Dequeue a nybble from the queue and store it in variable v.
>v Pop a nybble from the stack and store it in variable v.
?v Input a nybble and store it in variable v.
!n Output the nybble n.
@ End execution.
:d Define the label d.
#l Jump to a label. If you jump to a label that doesn't exist, it is an error.
%l Jump to a label, only if the stack/queue is empty.
+vn Add nybble n to the value of variable v (discarding all unused bits).
-vn Subtract nybble n from the value of variable v (discarding all unused bits).
^vn XOR nybble n to the value of variable v.
&vn NAND nybble n to the value of variable v.
[ ] Perform the commands inside the brackets using a new stack/queue, starting as empty. (The old stack/queue is restored at the end.) You may not jump into or out of a set of brackets.
[ ]d You may put a label name at the end of a set of brackets. A jump to the label acts as a subroutine, returning after executing the commands inside the brackets.
| (Inside a set of brackets) Replace the stack/queue with what it was just before the end of the previous execution of this set of brackets.
$ Swap variables X and Y.
~vl Divide value of v by 2, if remainder then goto label l.

Copy input to output:

:0?X!X#0

Hello World:

!48656C6C6F20576F726C6421@

Input a character output character before it in ASCII set:

?X?Y-Y1#Y
:0:1:2:3:4:5:6:7:8:9:A:B:C:D:E!XY@
:F-X1!XY@

Another shorter example to do the same as previous one:

?X?Y*X~X1~X1~X1~X1<X-X1*X:1>X-Y1!XY@