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\section{Shapes and sequences}
The idea is to support representations that make the best possible use
of the bits in the machine.
We have an initial or prefix sequence of tokens that are always used, followed
by a list of alternative list of possible suffix sequences.
When there are multiple alternatives, we need a tag.
We need to be able to mix sequences from either direction, too.
Finally, when possible we try to cram everything into  one machine
word.