In the following example, the SHOW FIELD statement shows the
    SEX field, which contains a VALID IF clause. The CHANGE FIELD
    statement removes the VALID_IF clause from the SEX field:

    RDO> DATA FILE DB$:PERSONNEL
    RDO> SHOW FIELD SEX
         SEX                              text size is   1
       Description:         M, F
       Missing value:       "?"
       Valid:               IF SEX = 'M' OR SEX = 'F' OR
                                    SEX MISSING
    RDO> CHANGE FIELD SEX NO VALID IF.
    RDO> SHOW FIELD SEX
         SEX                              text size is   1
       Description:         M, F
       Missing value:       "?"
    RDO> ROLLBACK