code: public static <T> DiffResult<T> diff(@Nonnull SheetReader<T> a, @Nonnull SheetReader<T> b) {
      return diff(a, b, null);
    }
docs: Returns the difference between two sheets. Note that T must implement
  hashCode() and equals() if you wish to have meaningful symmetric difference
  results.
  
  @param a the first sheet
  @param b the second sheet
  @return DiffResult object which holds the diff result