I need to identify (e.g. "alert") travel activities occuring after a work
activity for each unique travel event. My data looks like this...

Event Activity Start End
C090721 Travel alert 8/4/2009 16:59 8/4/2009 17:00
C090721 Work 8/4/2009 16:47 8/4/2009 16:59
C090721 Travel 8/4/2009 16:46 8/4/2009 16:47
C090722 Travel alert 8/3/2009 12:59 8/3/2009 13:00
C090722 Work 8/3/2009 11:00 8/3/2009 12:59
C090722 Travel 8/3/2009 8:00 8/3/2009 11:00

For travel event, C090721 the travel occuring after the work activity is
identified with "alert"