anchor.diff:  dropConstraintClause
       : DROP
       (
  -        (constraintPrimaryOrUnique CASCADE? (( KEEP | DROP) INDEX)?)
  -       | (CONSTRAINT constraintName ( CASCADE )?)
  +        constraintPrimaryOrUnique CASCADE? ((KEEP | DROP) INDEX)?
  +       | (CONSTRAINT constraintName CASCADE?)
       ) 
       ;
   
   alterExternalTable
  -    : ( addColumn
  -    | modifyColumn
  -    | dropColumn
  -    )+
  +    : (addColumn | modifyColumn | dropColumn)+
       ;
  
positive.code: import OracleKeyword, DataType, Keyword,OracleCreateIndex, OracleTableBase,OracleBase,BaseRule,Symbol;
  
  alterTable
      : ALTER TABLE tableName
       ( alterTableProperties
       | columnClauses
       | constraintClauses
       | alterExternalTable
      )?
      ;
      
  alterTableProperties
      : renameTable
      | REKEY encryptionSpec
      ;
  
  renameTable:
  
negative.code:     ;
  
  constraintClauses
      : addConstraintClause | modifyConstraintClause | renameConstraintClause | dropConstraintClause+
      ;
  
  addConstraintClause
  