The calculus of spans.

A span is a `<doc,startPosition,endPosition>` tuple.

The following span query operators are implemented:

- A SpanTermQuery matches all spans containing a particular Term.
- A SpanNearQuery matches spans which occur near one another, and can be used to implement things like phrase search (when constructed from SpanTermQueries and inter-phrase proximity (when constructed from other SpanNearQueries.
- A SpanOrQuery merges spans from a number of other SpanQueries.
- A SpanNotQuery removes spans matching one SpanQuery which overlap another. This can be used, e.g., to implement within-paragraph search.
- A SpanFirstQuery matches spans matching `q` whose end position is less than `n`. This can be used to constrain matches to the first part of the document.

In all cases, output spans are minimally inclusive. In other words, a span formed by matching a span in x and y starts at the lesser of the two starts and ends at the greater of the two ends.

For example, a span query which matches "John Kerry" within ten words of "George Bush" within the first 100 words of the document could be constructed with:

    SpanQuery john   = new SpanTermQuery(new Term("content", "john"));
    SpanQuery kerry  = new SpanTermQuery(new Term("content", "kerry"));
    SpanQuery george = new SpanTermQuery(new Term("content", "george"));
    SpanQuery bush   = new SpanTermQuery(new Term("content", "bush"));

    SpanQuery johnKerry =
       new SpanNearQuery(new SpanQuery[] {john, kerry}, 0, true);

    SpanQuery georgeBush =
       new SpanNearQuery(new SpanQuery[] {george, bush}, 0, true);

    SpanQuery johnKerryNearGeorgeBush =
       new SpanNearQuery(new SpanQuery[] {johnKerry, georgeBush}, 10, false);

    SpanQuery johnKerryNearGeorgeBushAtStart =
       new SpanFirstQuery(johnKerryNearGeorgeBush, 100);

Span queries may be freely intermixed with other Lucene queries. So, for example, the above query can be restricted to documents which also use the word "iraq" with:

    Query query = new BooleanQuery();
    query.add(johnKerryNearGeorgeBushAtStart, true, false);
    query.add(new TermQuery("content", "iraq"), true, false);