module bound  
! mark helix filter outputs where input is off data.
  use cartesian
  use helicon
  use regrid
  implicit none
  contains 
  subroutine boundn ( nold, nd, na, aa)
    integer, dimension( :), intent( in) :: nold, nd, na     ! (ndim)
    type( filter)                       :: aa
    integer, dimension( size( nd))      :: nb, ii
    real,    dimension( :), allocatable :: xx, yy
    integer                             :: iy, my, ib, mb, stat
    nb = nd + 2*na
    mb = product( nb)      ! nb is a bigger space to pad into.
    allocate( xx( mb), yy( mb))		     ! two large spaces, equal size
    xx = 0.				     !              zeros
    do ib = 1, mb  
! surround the zeros with many ones
      call line2cart( nb, ib, ii)           ! ii( ib)
      if ( any( ii <= na  .or.  ii > nb-na)) then
        xx( ib) = 1. 
      end if
    end do 
    call helicon_init( aa)		     ! give aa pointer to helicon.lop
    call regridn( nold, nb, aa)
    aa%flt = 1.		! put all 1's in filter
    stat = helicon_lop( .false., .false., xx, yy)	! apply filter
    call regridn(   nb, nd, aa)
    aa%flt = 0.	 ! remake filter for orig data.
    my = product( nd)
    allocate( aa%mis( my))		! attach missing designation to y_filter
    do iy = 1, my  
! map from unpadded to padded space
      call line2cart( nd, iy, ii )
      call cart2line( nb,     ii+na, ib )	    ! ib( iy)
      aa%mis( iy) =             ( yy( ib) > 0.)    
      ! true where inputs missing
    end do 
    deallocate( xx, yy)
  end subroutine 
end module