The first form of this command establishes a script
to be invoked when a data late fragment is received. The script
has appended to it the source id of the
data source that contributed the fragment, the
timestamp of the late fragment, and
the newest timestamp emitted.
If the script is an empty string, the current
script will no longer receive these events. If there is no prior
script this is a no-op.
The second form of the command does nothing.
Both forms return the script associated with these events prior to the command execution. If no script was associated with this event, both forms return an empty strig.