Concertlina II: Full Circle

I've noted earlier that I felt I had been going around in circles with
Concertina II, changing the instruction format back and forth, instead
of making progress to flesh it out.
Recently, I added a new instruction to facilitate looping.
But the trouble was that it took up tooo much opcode space.
One thing that occured to me was that if I went back to an old method
of specifying instructions longer than 32-bits: using a 4-bit pSupp
field to point into the same reserved area in the block as used for
pseudo-immediates, that would suit this instruction very well.

The reason is that if that techique were used, then I could use the
header that's also an instruction to just squeeze in the three-bit
decode field, and so access to the Loop instruction would be easy as
befits its importance.

Then I went back, and looked up an older version of Concertina II
which had it. It had complicated block headers. But worse than that,
it had _four_ different versions of the complete instruction set!
Which version was used depended on the header.The idea, of course,
that some headers required a pared-down version of the instruction set
so as to squeeze in more stuff.
It was also interesting to see how much further along I had gotten in
fleshing out that older version of the instruction set.

John Savard