Multiple flow() functions in same content property with difference ranges can cause problems

[bertfrees]

When a flow(x) is combined with a flow(y, volume) in the content property of a volume area, they can end up in a single dynamic-sequence, which results in flow(x) not being rendered if flow(y, volume) evaluates to nothing.

In some cases this is desired, in some cases not.

An example of where this is desired is in nlb-default.scss:

@volume {
    @end {
        content: flow(endnotes-title) flow(endnotes, volume);
    }
}