#$=head1 NAME
#$
#$fold - fold the ends inside
#$
#$=head1 SYNOPSIS
#$
#$C<ierr=fold_lop(adj,add,x,y)>
#$
#$=head1 PARAMETERS
#$
#$=over 4
#$
#$=item adj,add,x,y  - 
#$
#$      Standard operator interface
#$
#$=back
#$
#$=head1 DESCRIPTION
#$
#$Fold portion of y outside x back in on itself
#$
#$=head1 SEE ALSO
#$
#$L<triangle>
#$
#$=head1 LIBRARY
#$
#$B<geef90>
#$
#$=cut
module fold { # fold the ends inside
#% _lop (x, y)
    integer                             :: nu, nr, np, nt
    nu = size (y) 
    nt = size (x)
    np = (nt + nu) /2
    nr = (nt - nu) /2 + 1
    if (adj) {
       x (nt:np+1:-1)      = y (nu-nt+np+1:nu  ) + x (nt:np+1:-1)
       x (nr-1: 1:-1)      = y (         1:nr-1) + x (nr-1: 1:-1) 
       x (nr:np)           = y                   + x (nr:np)
    } else {
       y                   = y                   + x (nr:np)
       y (         1:nr-1) = y (         1:nr-1) + x (nr-1: 1:-1) 
       y (nu-nt+np+1:nu  ) = y (nu-nt+np+1:nu  ) + x (nt:np+1:-1)
    }
}