#$
#$=head1 NAME
#$
#$helicon - convolution using helix filters
#$
#$=head1 SYNOPSIS
#$
#$Initializer - C<call helicon_init(aa)>
#$
#$Operator    - C<ierr=helicon_lop(adj,add,xx,yy)>
#$
#$=head1 PARAMETERS
#$
#$=over 4
#$
#$=item aa - type(filter)  
#$
#$      helix filter to perform convolution with
#$
#$=item adj,add,xx,yy - 
#$
#$      standard operators parameters
#$
#$=back
#$
#$=head1 DESCRIPTION
#$
#$ Convolution, inverse to deconvolution.
#$ Requires the filter be causal with an implicit "1." at the onset.
#$
#$
#$=head1 SEE ALSO
#$
#$L<helix>
#$
#$=head1 LIBRARY
#$
#$B<geef90>
#$
#$=cut
#$
module helicon {                       # Convolution, inverse to deconvolution.
#             Requires the filter be causal with an implicit "1." at the onset.
use helix
type( filter) :: aa
#%  _init( aa)
#%  _lop ( xx,  yy)
integer iy, ix, ia
if( adj)              # zero lag
	xx += yy
else
	yy += xx
do ia = 1, size( aa%lag) {
      do iy = 1  + aa%lag( ia), size( yy) {    	
	    if( associated( aa%mis)) { if( aa%mis( iy)) cycle}
            ix = iy - aa%lag( ia)
            if( adj)
                        xx(ix) += yy(iy) * aa%flt(ia)
	    else	
                        yy(iy) += xx(ix) * aa%flt(ia)
	    }
      }
}