[developers] Characterisation in cheap

[Stephan Oepen]

hi again,

> trying to add characterisation to HaG, I experienced some surprising
> behaviour with cheap -t. 
> 
> Currently, there is no characterisation on GPREDs, regardless of whether
> they are set within the grammar or not. LKB by contrast appears to
> honour GPREDs set in the grammar.

what exactly do you mean by: set in the grammar or not?

> To test with the ERG, try 
> 
> echo "John, Paul, and George  barked." | \
> cheap -t -cm -nsolutions=1 -mrs  english
> 
> You'll find there is no characterisation on the implicit_conj_rel.

once more, i think you are pointing out a genuine bug.  but this is a
problem in the ERG, not in PET.  in the new universe, the assumption is
that characterization is controlled by the grammar, i.e. there is FROM
and TO information on the token FSs; lexical items copy these into an
appropriate place in the signs (`ORTH.FROM' and `ORTH.TO' in the ERG);
all rules make sure to percolate correct values along the periphery of
phrases; and any sign introducing a new EP co-indexes CFROM and CTO on
the PRED with its own FROM and TO value.  dan felt that it was not very
difficult adding co-references to implement this strategy, as existing
abstractions in the type hierarchy were suitable.  however, it appears
there is a missing co-reference on the sign introducing `implicit_conj'
here.
 
> Setting or unsetting MRS-CFROM-PATH etc. seems to have no effect.

the ERG deliberately does not set `mrs-cfrom-path' and `mrs-cto-path'.
these activate the procedural characterization mechanism in PET, and i
sincerely hope all code that destructively modifies feature structures
`behind the scenes' in the parser is obsoleted with chart mapping.  in
the not too distant future, we should try spring cleaning the PET code
base for removal of such legacy mechanisms ...

                                                     all best  -  oe

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+++ Universitetet i Oslo (IFI); Boks 1080 Blindern; 0316 Oslo; (+47) 2284 0125
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+++       --- oe at ifi.uio.no; oe at csli.stanford.edu; stephan at oepen.net ---
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