avoiding incorrect code

[Barry Smith]

    From the PETSc developers guide

  Do not use {\em if (rank == 0)} or {\em if (v == PETSC\_NULL)} or  
{\em if (flg == PETSC\_TRUE)} or {\em if (flg == PETSC\_FALSE)}
instead use {\em if (!rank)} or {\em if (!v)} or {\em if (flg)} or  
{\em if (!flg)}.

   There was a flag == PETSC_TRUE in PETSc 3.0.0 that wasted a lot of  
several peoples time finding it. Come on folks, we have better things  
to do with our time. Please avoid this incorrect usage.

    Barry