all code shares a common namespace, so variables defined in one tab can
be used in another during the whole fgfs session
- make code persistent (saved in autosave.xml)
- better error messages (code borrowed from nasal/lib/interactive.nas)
- clear button (replaces close button, which is now on the title bar)
- help button (explains how to add more tab buttons, etc.)
leave edit field!, press <Return> to execute; as an example try
screen.log.write("hello world"), or f16.canopy.open() in the F16,
or bo105.doors[3].move(0.3) in the Bo105 etc.
