Just some thoughts.
In other systems I've seen, people are using n#, where # is the slop
factor. There could be an extended name and a minimal name NEAR5 or n5.
I like the term near vs. within for this use.
I think that the term within could be used to specify a field. For
example, ("Doug Cutting" within author). This would search for the
phrase Doug Cutting in the field author.
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That said, I wouldn't mind seeing a syntax for modifying the
slop factor in
a phrase query ("Lucene" within 10 words of "search") :)
I agree that this is a big hole in the query language. Any ideas for
syntax?