# 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