module bound {          # mark helix filter outputs where input is off data.
use cartesian
use helicon
use regrid
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))   xx( ib) = 1. 
       }
    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
       }
    deallocate( xx, yy)
    } 
}