program laplace
code:  LaPlace solver sample program 
code:    From "getting Started in CM Fortran".
code:    Solution of LaPlace's equation on unit square.
code:    BC: f(x,1)=1, f(0,y)=f(x,0)=f(1,y)=2, 0<{x,y}<1.
code:    Initial interior starting iterate: f(x,y) = 0.
comment:  Need to include CMF timer utilities:
      include'/usr/include/cm/timer-fort.h'
      parameter(maxx=32,maxy=maxx)
      real f(maxx,maxy),df(maxx,maxy)
      real rms_error,max_error
      logical cmask(maxx,maxy)
      integer iteration
comment:  Clear and start timer 0.
      call cm_timer_clear(0)
      call cm_timer_start(0)
comment:  Initialize the mask for the interior points.
      cmask = .false.
      cmask(2:maxx-1,2:maxy-1) = .true.
comment:  Initialize f
      f = 2.
      f(:,maxy) = 1.
      where (cmask) f = 0.
comment:  Set dummy value for max_error.
      max_error = 1.
      iteration = 0
comment:  Iterate until max_error < 1.e-3 stopping criterion.
      do while (max_error.gt.1.e-3)
         iteration = iteration + 1
comment:  Compute df, the change at each iteration, and udate.
         df = 0.
         where (cmask)
	    df = 0.25*(cshift(f,1,1) + cshift(f,1,-1)
     &         + cshift(f,2,1) + cshift(f,2,-1)) - f
	    f = f + df
         endwhere
comment:  Compute RMS and Maximum errors.
         rms_error = sqrt(sum(df*df)/((maxx-2)*(maxy-2)))
         max_error = maxval(df,mask=cmask)
comment:  Output errors every 10th iteration
         if(mod(iteration,10).eq.0) then
	    write(6,*) iteration,rms_error,max_error
         endif
      enddo
comment:  Stop and Print timer
      call cm_timer_stop(0)
      print*,'total CM5 time'
      call cm_timer_print(0)
comment:  Output final iteration count and errors (stopping criteria).
      write(6,*) iteration,rms_error,max_error
      stop
      end