* Ensure valid output and update the "current node", if the base node is
the "current node".
If the base node is the state machine's "current node", we can safely assume
that it is attached to the document and check if sections are valid. We can
also update the "current node" of the parent state machine(s) after parsing.
This prevents invalid or mixed up document trees.
* Make the "current node" the default value for the "node" argument.
(This allows simple use for content included from external sources.)
* Restore `memo.section_level` after nested parsing into a custom node.
The calling code must ensure that section_level and current node stay in sync.
* Update unit tests.
