field <str> is referenced in COMPUTED BY field <str>

  Explanation:  You attempted to delete a field from  a  relation,
  but  the  field  is  used  in  a  COMPUTED  BY field in the same
  relation.

  User Action:   First,  review  the  database  design;  then,  if
  desired, delete the COMPUTED BY field, and then delete the field
  from the relation.