+
+ # this might move before $out to recalculate stats on source dataset?
+ my $c = { map { $_ => 1 } @columns };
+ #warn "XXX c ",dump( $c );
+
+ __path_modified( $path, 2 );
+ $o->{$_}-- foreach keys %{ $self->_loaded('stats') };
+ #warn "XXX o ",dump( $o );
+ my @added_columns = grep { $o->{$_} && ! $c->{$_} } keys %$o;
+ warn "# added_columns ",dump( @added_columns );
+ unshift @columns, @added_columns;
+
+ $self->session('columns', [ @columns ]);
+ $loaded->{$path}->{columns} = [ @columns ];
+ warn "# new columns ",dump( @columns );