[developers] Stack overflow in generator unpack-edge

[Dan Flickinger]

Using the current SVN versions of the ERG and the LKB, I get a stack overflow error resulting from a tight self-calling loop in the generator's unpack-edge!!() when generating (with packing) from the expletive-it subject (NP-X) reading of the sentence 
  It fails to rain.
It seems that the generator may be having difficulty distinguishing the edge "to rain" from the edge "for it to rain", both of which have identical semantics.

  Dan