PARTIALLY WORKING - except multiplexing_stexLineInsideMarkdownCombine.
DEBUG NOISE.
TODOC.

[multiplex] Option to combine outer mode's style with inner.
Outer mode sees placeholder text (rather than just newline) and the style it returns for this text is added to all inner tokens.
refactoring wip. BACK TO ORIG BEHAVIOR
All tests pass!  DEBUG NOISE, TODOC.
test comment.
untabify
clean, comment.
Flattened logic with conditional swallowing of token into outerStyle.
Probably uglier.
WIP refactor