support for pretty blocks that are "open" and thus have no impact on formatting, only on markup;
effectively revert 2cb4a2970941 and unify pretty-printing in Scala and ML, following Scala before that change;
build error messages are properly formatted again (amending 320bcbf34849): "no_report" markup does not affect its enclosed line break anymore;
(* Title: Pure/Sessions.thy
Author: Makarius
PIDE markup for session ROOT.
*)
theory Sessions
imports Pure
keywords "chapter_definition" "session" :: thy_decl
and "description" "directories" "options" "sessions" "theories"
"document_theories" "document_files" "export_files" :: quasi_command
and "global"
begin
ML \<open>
Outer_Syntax.command \<^command_keyword>\<open>chapter_definition\<close> "PIDE markup for session ROOT"
Sessions.chapter_definition_parser;
Outer_Syntax.command \<^command_keyword>\<open>session\<close> "PIDE markup for session ROOT"
Sessions.session_parser;
\<close>
end