Creates a constraint for a relation or relations. A constraint
    defines a set of conditions that restrict the values stored in
    relations. When you store and modify field values, the constraint
    checks the validity of the values and generates an error message
    if the constraint is violated.

    Example:

       RDO>  DEFINE CONSTRAINT EMPLOYEE_ID_REQUIRED
       cont>  FOR E IN EMPLOYEES
       cont>  REQUIRE E.EMPLOYEE_ID NOT MISSING.