code: @Override
      public StrBuilder append(final CharSequence seq) {
          if (seq == null) {
              return appendNull();
          }
          return append(seq.toString());
      }
docs: Appends a CharSequence to this string builder.
  Appending null will call {@link #appendNull()}.
  
  @param seq  the CharSequence to append
  @return this, to enable chaining
  @since 3.0