Select one union arm or optional value

Encode the discriminant and only its selected arm; void contributes zero bytes.

• A discriminated union starts with an int, unsigned int, enum or boolean discriminant, then encodes just the selected arm. A default arm is optional. Without one, an unspecified discriminant has no valid union encoding; each case must itself be a legal value of the discriminant’s type (§§4.15,6.4).
• Chosen example: union reply switch (bool ok) { case TRUE: unsigned int value; case FALSE: void; };. TRUE with value 7 encodes two words:1and7. FALSE encodes only the four-byte zero discriminant, because void contributes zero bytes (§§4.4,4.15–4.16).
• The unknown-case state is a SEPARATE int-switch example with cases 0 and 1 and no default. Discriminant3 has no selected arm, so the drawing stops. An int-switch with default:void could encode 3 with no arm bytes; that default would not make an invalid boolean or unassigned enum value legal (§§4.3–4.4,4.15,6.4).
• Optional data type-name *name is shorthand for a boolean union: FALSE followed by void or TRUE followed by the encoded element. It is also equivalent to a counted array of at most one element. The star does not serialize a process address, ownership pointer or memory-layout offset (§4.19).
• For optional string cat, present TRUE adds word 1 before the eight-byte counted string: twelve bytes. Absent FALSE is only word 0: four bytes. Recursive structures built this way require safe processing limits; no default maximum recursion depth is specified by XDR (§§4.19,8).