code: public CollectionSchemaUpdate set(String key, IOperation operation) {
      collectionUpdateData.put(key, operation);
      return this;
    }
docs: A method to set a new Operation for a key. It may be of type ADD, RENAME or DELETE.
  Only one operation per key can be specified. Attempt to add a second operation for a any key will override the first one.
  Attempt to add a ADD operation for a key which already exists will have no effect.
  Attempt to add a DELETE operation for akey which does not exist will have no effect.
  
  @param key (a.k.a JSON Field name) for which operation is being added
  @param operation  operation to perform
  @return  the updated CollectionSchemaUpdate