What is the DFDL string literal syntax for a hexBinary type value?

E.g., I want a hex binary whose value is the 4 bytes described by this hex: DE AD BE EF.

<element name="myHexBin" type="xs:hexBinary" dfdl:inputValueCalc="{ ... }"/>

So, what can one syntactically put, for literal constant values, in the input value calculation expression?

Note that this is legal pure (non-DFDL) XSD (I think)

<element name="aHexBin" type="xs:hexBinary" fixed="DeadBeef"/>

That is, the fixed/default are allowed and one specifies these values as just strings of hex digits. Notice no special escaping or anything. You just use a string that just so happens to contain hex digits.

I think there are three possibilites
(a) we allow "DEADBEEF" i.e., because the type of the expression is hexBinary, a string is cast to hexBinary by interpreting it as hex nibbles.
(b) we require a special kind of string literal - a bytes-only string literal, so for example: "%#rDE;%#rAD;%#rBE;%#rEF;" is the way you create 4 bytes. If you just put characters, then that's a processing error - like a cast failure. Only raw-bytes allowed.
(c) Anything you return from the expression is converted to a hexBinary by unparsing it to bytes (using current properties), then using the bytes as the hexBinary data. So you could have an expression that returns a double, and that would create 8 bytes if representation="binary". In this case the decimal number 3735928559 (hex 0xdeadbeef) as a binary bigEndian int would produce the 4 bytes I want.