> I don't presume to know -- it's just the first system I came into contact with which has this concept.

It was already present on the Xerox PARC systems.

You can see a little bit of it here about Symbiotics, http://www.loper-os.org/?p=932 at minute at minute 30.

Or here for Smalltalk-80, https://www.youtube.com/watch?v=JLPiMl8XUKU