SUBROUTINE  converge(  u,  gdiff  )
      USE numerics
      IMPLICIT  NONE
      REAL(rp),  INTENT(OUT)   ::  gdiff
      REAL(rp),  DIMENSION(  sx-1:ex+1,  sy-1:ey+1,  sz-1:ez+1,  0:1),  INTENT(IN)  ::  u
      END  SUBROUTINE  converge