[developers] bi-directional subsumption test

[Stephan Oepen]

hi john,

as i am obviously approaching senility, let me use email for permanent
storage.  working with woodley last night, we re-discovered a problem
in the bi-directional subsumption test that ulrich and i had addressed 
in PET only a few years ago:

  Change 601 by oe at oe on 2003/06/01 02:19:17

    Finally! Fix bug in subsumption test related to subgraph sharing: since
    we may end up having the same dag in both input structures (which cannot
    happen for unification during parsing, as the chart adjacency condition
    rules out combinations of edges with joint daughters).  Using the copy
    slot to record the `corresponding' node for re-entrant nodes during
    subsumption in both directions, means we may hallucinate coreferences
    when we effectively visit the same dag as part of both input dags.
    The fix was identified on the rooftop of 601 4th Street: use separate
    slots for forward and backward correspondents; assume that when we call
    the subsumption test it is save to use the forward slot.

  Affected files ...

  ... //pet/main/cheap/dag-tomabechi.cpp#15 edit

  Differences ...

  ==== //pet/main/cheap/dag-tomabechi.cpp#15 (text) ====

  660c661
  <     dag_node *c1 = dag_get_copy(dag1),
  ---
  >     dag_node *c1 = dag_get_forward(dag1),
  682c683
  <             dag_set_copy(dag1, dag2);
  ---
  >             dag_set_forward(dag1, dag2);

to my dismay i see no records of emailing people about this earlier.  i
would suggest making the same change in the LKB now.

                                            what do you think?  -  oe

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