IWriter is the interface required by History to process execution history events. (see History). This interface is
also used by Cache to write events to the cache file.
As used by History, IWriter.write() must be prepared to accept Hyades ExecutionEvent objects and String objects.
When a new history is opened the String "<EXECUTION>" is written via IWriter.write() and when it is closed the String
"</EXECUTION>" is writen via IWriter.write(). Otherwise, non-cached events are written as ExecutionEvent objects
and cached events are written as String objects.