Presentation -- support structuring above slide level

[kvistgaard]

Currently, the allowed structure is:

• {{slides}}
  • {slide}

So there is no way to organize slides or simply turn an existing page into a presentation without "flattening" at the slide level

It would be nice to have

• {{slides}}
  • {section} ((not visible as slide unless it is {title}
    • {slide}

It would be nice if there are many levels of sections if there is a way to do it without too much effort to show which is the slide level.

[dvargas92495]

Moved to #745