more accurate indentation: retain (before: Double) until it is materialized as blanks;

misc tuning: more uniform;

clarified internal data representation, following push/pop model of Scala version;

tuned names;

more accurate formatting of open_block: markup only, without affecting layout (e.g. via force_next); tuned signature;

tuned: more uniform;

proper treatment of XML.Wrapped_Elem as open_block (amending 7cacedbddba7, but this case is presently unused);

tuned;