Creates a trigger for the specified relation. A trigger is
    a mechanism which associates a set of rules with an update
    operation.

    Example:

    RDO> DEFINE TRIGGER EMPLOYEE_ID_CASCADE_DELETE  <--- trigger name
    cont>        BEFORE ERASE                       <--- action time
    cont>        FOR E IN EMPLOYEES            <--- subject relation
    cont>            EXECUTE
    cont>                FOR D IN DEGREES WITH <--- triggered action rse
    cont>                  D.EMPLOYEE_ID = E.EMPLOYEE_ID
    cont>                  ERASE D             <--- triggered action
    cont>                END_FOR
    cont>            FOR EACH RECORD.          <--- frequency clause