static {
   xstream.alias("exec-step", StepCommand.class); // $NON-NLS-1$
   BreakpointData.configureXStream(xstream);
 }
 static {
   xstream.alias("exec-continue", ContinueCommand.class); // $NON-NLS-1$
   BreakpointData.configureXStream(xstream);
 }