module hconest {             # masked helix convolution, adjoint is the filter.
use helix
  real, dimension (:), pointer :: x
  type( filter)                :: aa
#% _init( x, aa)
#% _lop( a, y)
    integer  ia, ix, iy
    do ia = 1, size( a) {
         do iy = 1  + aa%lag( ia), size( y) {    if( aa%mis( iy)) cycle  
            ix = iy - aa%lag( ia)
             if( adj)    a( ia) +=  y( iy) * x( ix)
             else        y( iy) +=  a( ia) * x( ix)
         }
    }
}