Gramlib syntax error messages are less precise, sometimes wrong (Mantis 4329)

[damiendoligez]

An old bug report from the Mantis tracker. See http://caml.inria.fr/mantis/view.php?id=4329 for full details.

camlp4 grammars in 3.10 seem to give less precise syntax errors then they used to do: in a sequence of symbols, it complains about the beginning when the 2nd, 3rd, etc. symbol is wrong. When there are multiple possible sequences that start the same way, this can become very confusing.
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