anchor.diff:      ;
   
   constraintClauses
  -    : addConstraintClause
  -    | modifyConstraintClause
  -    | renameConstraintClause
  -    | dropConstraintClause+
  +    : addConstraintClause | modifyConstraintClause | renameConstraintClause | dropConstraintClause+
       ;
   
   addConstraintClause
  
positive.code: dropConstraintClause
      : DROP
      (
          (constraintPrimaryOrUnique CASCADE? (( KEEP | DROP) INDEX)?)
         | (CONSTRAINT constraintName ( CASCADE )?)
      ) 
      ;
  
  alterExternalTable
      : ( addColumn
      | modifyColumn
      | dropColumn
      )+
      ;
  
negative.code:     ;
  
  columnOrVirtualDefinitions
      : LP_ columnOrVirtualDefinition (COMMA columnOrVirtualDefinition)* RP_
      | columnOrVirtualDefinition
      ;
  
  columnOrVirtualDefinition
      : columnDefinition | virtualColumnDefinition
      ;
  
  modifyColumn
  