anchor.diff:      ;
   
   alterExternalTable:
  -    ( addColumnClause
  -    | modifyColumnClauses
  -    | dropColumnClause
  +    ( addColumn
  +    | modifyColumn
  +    | dropColumn
       | parallelClause
       | externalDataProperties
       | (REJECT LIMIT ( NUMBER | UNLIMITED ))
  
positive.code:     CHECKPOINT NUMBER
      ;
      
  renameColumnClause:
      RENAME COLUMN columnName TO 
      ;
      
  modifyCollectionRetrieval:
  
negative.code:      ;
   
  constraintPrimaryOrUnique:
        primaryKey
       | (UNIQUE columnList)
       ;
          
  