append
Returns the history stored with checkpoint added, for a Checkpointer that keeps the checkpoints of a thread in one string. checkpoint replaces the checkpoints with its step or a later one, as Checkpointer.save asks, and of the result the last maxHistory are kept.
The states of the stored checkpoints are not read. A line of stored that is not a checkpoint is left out, so that a damaged history does not stop a run from being saved.