Invoked before indexing a Field instance if terms have already been added to that field. This allows custom analyzers to place an automatic position increment gap between Field instances using the same field name. The default value position increment gap is 0. With a 0 position increment gap and the typical default token position increment of 1, all terms in a field, including across Field instances, are in successive positions, allowing exact PhraseQuery matches, for instance, across Field instance boundaries.
position increment gap, added to the next token emitted from {@link #TokenStream(String,Reader)}