Requests that the datasource should be moved to the next row. An advance
operation does not change the current cursor position. The cursor is not
moved until a 'COMMIT' operation has been reached.
Repeatable sections will perform an auto-commit based on the group in which
they are in.
Finishes (and closes) the currently open context. If the last mark has been
closed, the datasource is also closed.
If all datasources have been closes, the empty datasource is used. This
datasource cannot be closed (closing has no effect on it).