module regrid {         # convert a helix filter from one size data to another
use helix
use cartesian
contains
  subroutine regridn( nold, nnew, aa) {
    integer, dimension (:), intent (in) :: nold, nnew  # old and new helix grid
    type( filter)                       :: aa
    integer, dimension( size( nold))    :: ii
    integer                             :: i, h0, h1, h
    call cart2line( nold, nold/2, h0)	# lag of any near middle point on nold
    call cart2line( nnew, nold/2, h1)	# lag                          on nnew
    do i = 1, size( aa%lag) {		# forall given filter coefficients
       h = aa%lag( i) + h0		# what is this?
       call line2cart( nold, h, ii)	# 
       call cart2line( nnew,    ii, h)	# 
       aa%lag( i) = h - h1		# what is this
    }
  }
}