more uniform treatment of optional_argument for Latex elements; discontinued somewhat pointless element position in Isabelle/Scala: semantic add-ons are better provided in Isabelle/ML; clarified signature of class Latex: overridable unknown_elem allows to extend the markup language;

removed pointless 'text_cartouche' command: regular 'text' already supports cartouches;

example: alternative document headings, based on more general document output markup;

more general document output: enclosing markup is defined in user-space;