- filter sgmlenc: utilize doctype
- symlink filter
- finish implementing filter for file-system access (attributes, write, etc.)
- investigate whether overloading (man overload) should be used in API
- Add PlexTreeMutable and PlexTreeImmutable as subclasses of PlexTreeMem,
  and move all write methods to PlexTreeMutable. By convention,
  PlexTreeImmutable cursors refer only to immutable trees.
  PlexTreeMem should no longer be used directly (i.e., not have a
  "new" method). The idea being that when write methods accept
  PlexTreeMutable parameters to embed into a tree elsewhere, these will
  always have to be copied first, whereas PlexTreeImmutable objects
  can just be referenced. (This may not work, an immutable API
  ought to be purely functional; this may be more about refcounts!)
- text, meta, c, etc. should return immutable objects;
  use cm, metam, etc. for mutable ones
- editor syntax-highlighting/folding support, e.g.
  * https://code.visualstudio.com/api/language-extensions/syntax-highlight-guide

Known bugs:

none

