Of inconsistent depth for levels and embedded lists

Now I’m working on the tree rehierarchization, which means taking flat items and giving them a tree-like structure. This applies for embedded lists and levels. For a Novelang source document like this:

Blah.

== Depth 1

Boo.

=== Depth 2

Yuck.

The parser converts this into an almost flat structure where text preceded by a sequence of equal signs become level introducers. So we get this intermediary structure:

n:part
 +-- n:paragraph-regular "Blah."
 +-- level-introducer "==", "Depth 1"
 +-- n:paragraph-regular "Boo."
 +-- level-introducer "===", "Depth 2"
 +-- n:paragraph-regular "Yuck."

A rather hidden step is the hierarhizer which converts level introducers (which also convey indentation information) into plain levels. Hierarchizer’s result looks like this:

n:part
 +-- n:paragraph-regular "Blah."
 +-- n:level
      +-- n:level-title "Depth 1"
      +-- n:paragraph-regular "Boo."
      +-- n:level
           +-- n:level-title "Depth 2"
           +-- n:paragraph-regular "Yuck."

This all works the same with (yet unimplemented) embedded lists so we won’t have the discussion twice. Just keep in mind how embedded lists look like:

- depth 1
  - depth 2

Now what should happen if source document looks like this? See:

=== Depth 2

== Depth 1

More generally, what should happen when a level introducer has no preceding level introducer of a smaller depth? A tempting approach is promoting first item to the smallest depth. This looks smart but this would distort information. Example above becomes:

n:part
 +-- n:level
      +-- n:level-title "Depth 2"
 +-- n:level
      +-- n:level-title "Depth 1"

The hierarchizer could also create a level from nothing in order to keep the correct depth. This is information distortion as well:

n:part
 +-- n:level         // Created from nothing
      +-- n:level
           +-- n:level-title "Depth 2"
 +-- n:level
      +-- n:level-title "Depth 1"

Here is the real question: how could inconsistent level depth mean something? There could be some extreme cases when concatenating different Parts, but this should not pollute more general cases. Maybe the intent is to have a rendered document with small titles in some kind of introduction happening before a the big title of some kind of chapter. Then it should be handled at stylesheet level. Yet the cleanest and simplest approach for handling depth inconsistencies for levels and embedded lists is to spit an error.