fix, make echse(1) tool survive a DSO whose only event is a 0-event

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
fix, port 628c3a2 to echs-lisp tool

Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
minor, introduce instantaneous states and operator ITS() to emit them

Signed-off-by: Sebastian Freundt <hroptatyr@fresse.org>
Signed-off-by: Sebastian Freundt <freundt@ga-group.nl>
Merge branch 'tool/echse' into next

* tool/echse:
  fix, port 628c3a2 to echs-lisp tool
  fix, make echse(1) tool survive a DSO whose only event is a 0-event
Merge branch 'core/event' into next

* core/event:
  minor, introduce instantaneous states and operator ITS() to emit them