OTS11 — OTS-RTF: PropagationContext and is_same_transaction

otid is described as follows on p. 10-75 of ptc/00-10-03:

"otid
An identifier specific to the current transaction or subtransaction.
This value is intended to support efficient (local) execution of the
is_same_transaction and hash_transaction operations when the
importing Transaction Service does interposition".

However, it is never defined how the otid can be used to determine
when two transactions are the same:

• is comparing the "current" otids sufficient?
• or, when nested transactions are used, the otid, the parent's otid (etc.)
  need to be compared?