    # File lib/rvg/rvg.rb, line 193
            def background_fill_opacity=(opacity)
                warn "background_fill_opacity= has no effect in nested RVG objects" if @nested
                begin
                    @background_fill_opacity = Float(opacity)
                rescue ArgumentError
                    raise ArgumentError, "background_fill_opacity must be a number between 0 and 1 (#{opacity} given)"
                end
            end