In processing a source entity with a transformation entity, an action set forth
in a portion of the transformation entity is executed. A portion of the source
entity is the subject of this action. This action causes a particular portion of
a result entity to be generated. After this portion of the result entity is generated,
a set of history information is associated therewith, which comprises a reference
to the portion of the transformation entity that gave rise to the action, and a
reference to the portion of the source entity that was the subject of the action.
Given this information, it is possible to perform backtracking at a later time,
so that when this particular portion of the result entity is selected, those portions
of the transformation and source entities that gave rise to the particular portion
of the result entity can be indicated to a user.
