A Token is an occurence of a term from the text of a field. It consists of
a terms text, the start and end offset of the term in the text of the field,
and a type string.
The start and end offsets permit applications to re-associate a token with
its source text, e.g., to display highlighted query terms in a document
browser, or to show matching text fragments in a KWIC (KeyWord In Context)
The type is an interned string, assigned by a lexical analyzer
(a.k.a. tokenizer), naming the lexical or syntactic class that the token
belongs to. For example an end of sentence marker token might be implemented
with type eos. The default token type is word.