/// <summary> Return the offsetGap from the analyzer assigned to field </summary>
 public override int GetOffsetGap(Mono.Lucene.Net.Documents.Fieldable field)
 {
     Analyzer analyzer = (Analyzer)analyzerMap[field.Name()];
     if (analyzer == null)
         analyzer = defaultAnalyzer;
     return analyzer.GetOffsetGap(field);
 }