[torqueusers] pbs_nodefile parallel job mother superior node listed position reliable?

[David Golden]

Somewhat silly question, but it suddenly began to trouble me:
Is the first node listed in $PBS_NODEFILE in a parallel job *defined* to be 
the "mother superior" for the job, or  does it just happen to work out that 
way right now?

i.e. can I reasonably safely assume the first node will be the m.s. when 
processing $PBS_NODEFILE within a job, or should I go do something more 
involved to cross-check?