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This is a really old coroutine library I wrote for a collaborative EC
project named REX in the early 1990s.

Back in those days, there wasn't really any such thing as native threads
on Unix and this was a primitive way of achieving that. In fact it was
sufficient to implement an actor-style of message-passing which made
concurrent programming very simple and easily comprehensible to students.

This code was lost (by me) when I left Imperial College in the late 1990s.
Thanks to the power of Google, I discovered that it was available in at
least one place on the web --- as part of Berkeley's pSather OS at ICSI,
here: ftp://ftp.icsi.berkeley.edu/pub/sather/Sather/System/pSather/lwp/.
Claudio Fleiner at ICSI had extensively modified it in interesting ways
to suit their needs there.

I've stripped out Claudio's changes to restore it to its original condition
because once again I need coroutines, this time on Atmel microcontrollers
(such as those found on the Arduino). The main reason for this is that
coroutines would obviate the need for the inevitable state-machine that
is required to be implemented (badly) whenever anything complicated is
attempted on that platform.

So far, I've only tested this on Linux/x86: the other architectures probably
don't compile any more --- if you can even find them still!