Class represents a unit of text on sentence duplicity checking level
(see Constants.CHECK_DUPLICITY_LEVEL).
This class is *IMMUTABLE*, which means that each time a method that is supposed
to change something is called, another instance is returned. The objects of this
type are pooled using SentenceUnitIDPool so that only one
instance of SentenceUnitID exists for each document.
Returns textual representation of the unit to be printed in duplicity checking reports.
This is used for identifying units in the examined document and should not contain document id.
Returns textual representation of the unit to be printed in duplicity checking reports.
This is used for identifying units in the already indexed documents
and should contain document id.