Faire enough, so prepare for an history lesson. :)

Mesa/Cedar was created at Xerox PARC, as an extension of the Xerox Star 8010 system, which was programmed in Mesa, the language that inspired Niklaus Wirth to create Modula-2.

http://www.digibarn.com/collections/systems/xerox-8010/

https://www.youtube.com/watch?v=Cn4vC80Pv6Q

https://www.youtube.com/watch?v=ODZBL80JPqw Mesa evolved to Mesa/Cedar, which allowed for automatic memory management via reference counting and a tracing collector for the cycles.

https://archive.org/details/bitsavers_xeroxparcteCedarProgra...

The environment had a REPL and debugger inspired by the Smalltalk and Lisp systems at Xerox.

Think of Swift Playgrounds, kind of.

This system inspired Niklaus Wirth to create Oberon, which you can read about in his book, about how the OS was built.

http://www.ethoberon.ethz.ch/books.html

In 2013, Niklaus Wirth revised the language and updated the book for a FPGA based computer.

http://people.inf.ethz.ch/wirth/ProjectOberon/index.html

There were several generations of the Oberon OS, the last one before it stop being relevant, the Bluebottle OS coded in Active Oberon variant.

http://www.progtools.org/article.php?name=oberon&section=com...

It had a strange mix of UI, using a mix of text and mouse input. Basically all applications were dynamic modules and the procedures/functions with a special signature could be used from the UI.

So you could select text or objects from applications, and invoke such operations on the selection.

The Oberon System 3 and Oberon V4 with their Gadgets framework were the best experience, before AOS was designed.

Most relevant OS in Modula-3 was SPIN OS,

http://www-spin.cs.washington.edu/external/overview.html

There were a few others, this is just a small sample.