I have some questions on MPI_T_init_thread, which are unclear in the Standard.

 1) Must MPI_T_init_thread() be called in programs before using MPI_T?

 2) Must MPI_T_init_thread() be only called by the main thread?

 3) Is it wrong to do

     MPI_T_init_thread(MPI_THREAD_SINGLE, &provided);

     MPI_Init_thread(LEVEL_HIGHER_THAN_SINGLE, &provided);

    or vice versa,

     MPI_Init_thread(MPI_THREAD_SINGLE, &provided);