code: public final DataSet add(DataSet other) {
      checkIfNotReadOnly();
      if (other.getSource() != source) {
        throw new InvalidOperationException("Data source mismatch.");
      }
      rows.addAll(other.rows);
      return this;
    }
docs: Add rows of given data set to this data set.
  
  @param other The other data set.
  @return The data set instance (for chained calls).
  @throws InvalidOperationException
  if this data set is read-only, or
  if the data source of the other data set differs
  from this one.
  @see #row(Object...)
  @see #rows(Object[][])
  @see #isReadOnly()