[developers] Preprocessor, again

[Dan Flickinger]

As a follow-up to yesterday's message, note that a similar error (assigning
NIL as the span for a token) arises with the ERG preprocessor when one
token is replaced with another for the current cheap and cheerful spell 
correction.  For example, 
(lkb::preprocess "The chears arose.") 
gives the span of the spell-corrected "cheers" as NIL.  But note that if the 
misspelling is at the end of the input, the span is okay: 
(lkb::preprocess "The chears.")

  Dan