code: public MutationState add(Document... documents) {
          if (documents == null || documents.length == 0) {
              throw new IllegalArgumentException("At least one Document must be provided.");
          }
          for (Document d : documents) {
              addToken(d.mutationToken());
          }
          return this;
      }
docs: Add one or more {@link Document Documents} to this {@link MutationState}.
  
  @param documents the documents where the tokens are extracted from.
  @return the modified {@link MutationState}.