module heliarr { # an array of two helix convolutions
  use helicon
  type (filter) :: aa1, aa2
  integer       :: nx
#% _init (nx, aa1, aa2)
#% _lop (xx, yy)
  integer stat1, stat2
  call helicon_init (aa1)
  stat1 = helicon_lop (adj, .true., xx, yy (:nx))
  call helicon_init (aa2)
  stat2 = helicon_lop (adj, .true., xx, yy (1+nx:)) 
}