module misinput {	               # find a mask of missing filter inputs
  use helicon
contains
  subroutine find_mask( known, aa) {
    logical,  intent( in)          :: known(:)
    type( filter)                  :: aa
    real, dimension( size (known)) :: rr, dfre
    integer                        :: stat
    where( known) dfre = 0.    
    elsewhere     dfre = 1.    
    call helicon_init( aa)
    aa%flt = 1.
    stat = helicon_lop( .false., .false., dfre, rr)
    aa%flt = 0. 
    where ( rr > 0.)   aa%mis = .true.	
    }
}