On 18 Feb 2016, at 10:16, Hans Hagen <pragma@wxs.nl<mailto:pragma@wxs.nl>> wrote:

also, DEK used a keyboard with some special characters (probably dating from those assembler like computer languages) which is why the plain tex format has:

Could it have been a keyboard especially for the APL language? Just a wild guess, but I remember that language using a lot of such symbols.

Hans van der Meer