[expr] bit-wise "xor" operator

Arguments must be integers, result is an integer.

Bit ''n'' of the result is 1 if bit ''n'' of the two arguments differ. Otherwise, bit ''n'' of the result is 0.

To evaluate `$a^$b` when either `$a` or `$b` is negative, we make use of the following reasoning:

%| Case | Result |%
&| `$a>=0`, `$b>=0` | Bitwise operation |&
&| `$a>=0`, `$b<0`  | `$a^$b == ~($a ^ ~$b)`        Contrapositive law <<br>>`$a^$b == ~($a ^ (-1-$b))`    Extended definition of [~] <<br>> `$a^$b == -1-($a ^ (-1-$b))` Extended definition of [~] <<br>> Since `$a` and `(-1-$b)` are both non-negative, the [^] in the last expression can be evaluated in bitwise fashion. |&
&| `$a<0`, `$b>=0`  | Commute to ($b^$a) and evaluate as above. |&
&| `$a<0`, `$b<0`   | `$a^$b == (~$a) ^ (~$b)`     Contrapositive law <<br>> `$a^$b == (-1-$a) ^ (-1-$b)` Extended definition of [~] <<br>> Since `(-1-$a)` and `(-1-$b)` are both positive, the [^] in the last expression can be evaluated in bitwise fashion. |&

**Examples**

======
% expr 0b010 | 0b000
2
======

[[So, what do I need to add to this example so the result is binary as well?  
Some sort of [format] - but I don't see a binary conversion sequence in the docs...]]
[RS]: See for instance [to.binary]

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