It is currently installed to include/petsc/mpiuni/mpi.h and petscsys.h includes it as <mpi.h>, which means that users of MPIUNI need to put -I/prefix/include/petsc/mpiuni in their command lines. Matt and I agree that this is bad. We disagree on the solution. He wants to install it to /prefix/include/mpi.h as though the user had written --download-mpich. This would conflict if a user later installs a real MPI to that location. I would rather that petscsys.h include <petsc/mpiuni/mpi.h> because it can't be used without PETSc and nobody who ever wrote #include <mpi.h> in their own code will be happy if they get MPIUNI.