The Problem: The  MPI-1.1 standard says, in the discussion of  MPI_INTERCOMM_CREATE, both that  
  
  The groups must be disjoint  
  
and that  
  
  The leaders may be the same process.  
  
To further muddy the waters, the reason given for ``The groups must be  
disjoint'' is based on concerns about the implementation of  
 MPI_INTERCOMM_CREATE that are not applicable for the case where  
the leaders are the same process.  
 
The Fix: Delete the text:  
  
  (the two leaders could be the same process)  
  
from the discussion of  MPI_INTERCOMM_CREATE.  
 
Replace the text:  
  
All inter-communicator constructors are blocking and require that the local  
  and remote groups be disjoint.    
  
 
  
The groups must be disjoint for several reasons.  Primarily, this is the  
    intent of the intercommunicators --- to provide a communicator for  
    communication between disjoint groups.  This is reflected in the  
    definition of  MPI_INTERCOMM_MERGE, which allows the user to  
    control the ranking of the processes in the created intracommunicator;  
    this ranking makes little sense if the groups are not disjoint.  In  
    addition, the natural extension of collective operations to  
    intercommunicators makes the most sense when  
    the groups are disjoint.  
   ( End of advice to users.)   
 All inter-communicator constructors are blocking and require that the local  
 and remote groups be disjoint in order to avoid deadlock.   
  
with   
  
  
  
 
 Advice to users.