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for compatibility with both ocamlyacc and Menhir.

sentences and before the message.

Still remains to preserve comments inside a run, after the sentences and
before the message.

Used by --interpret-error, --list-errors and --update-errors.

Also simplified [Interpret] slightly to report just one syntax error.
[Segment] still drops # comments when in idle mode, this should be fixed.

Introduced a ## marker for auto-generated comments.

error from being displayed when [pos_fname] has not been initialized.