Consumers (i.e., IndexWriter) use this method to advance the stream to the next token. Implementing classes must implement this method and update the appropriate Util.Attributes with the attributes of the next token. The producer must make no assumptions about the attributes after the method has been returned: the caller may arbitrarily change it. If the producer needs to preserve the state for subsequent calls, it can use AttributeSource.CaptureState to create a copy of the current attribute state. This method is called for every token of a document, so an efficient implementation is crucial for good performance. To avoid calls to AttributeSource.AddAttribute(Type) and AttributeSource.GetAttribute(Type), references to all Util.Attributes that this stream uses should be retrieved during instantiation. To ensure that filters and consumers know which attributes are available, the attributes must be added during instantiation. Filters and consumers are not required to check for availability of attributes in IncrementToken().
