The KEEPONE_PROPERTY is an extension to the KEEP_PROPERTY, that provides a hint
to the progress UI to ensure that only a single Job of a Job family is kept in the
set of kept Jobs.
Update flag constant (bit mask value 2) indicating that the operation
should maintain local history by taking snapshots of the contents of
files just before being overwritten or deleted.
A KeySequence is defined as a list of zero or more
KeyStrokes, with the stipulation that all
KeyStroke objects must be complete, save for the last one,
whose completeness is optional.