Now supported by the table back-end.

Only in internal form, for now -- does not exist in concrete syntax.
Not yet supported by the back-ends.

Cleanup in [NonTerminalDefinitionInlining], and the beginning of a new way of translating positions when %inlining. Unfinished.

and [rename_sw_inner] to [Nonterminaldefinitioninlining], where they
belong.

$startpos(x) in the callee if the callee itself happened to be called x.

Because no keyword appears twice in a set of keywords.

It passes.
This will allow some serious simplification.

This has to do with 1- updated locations produced by Menhir's bootstrapped
parser; and 2- the cleanup in [Action] re-orders some bindings for the
position keywords.

Updated the code back-end to produce the same positions as ocamlyacc and the table back-end, in principle.
This involves:
1- moving the [endp] field (when it exists) to offset 1 in every stack cell,
   so it can be found when reducing an epsilon production;
2- creating a sentinel cell with an [endp] field at offset 1;
3- changing the code for epsilon productions to read the [endp] field in the
   top stack cell.

Updated [Invariant] with the constraint that an end position must exist on top of the stack whenever an epsilon production is about to be reduced.