The type for character folder. The integer is the index in the string where the `Uchar or `Malformed starts.

fold e ?off ?len f a s is f ( ... (f (f a pos u₀) j₁ u₁) ... ) ... ) j_n u_n where u_i, j_i are characters and their start position in the e encoded substring s starting at pos and len long. The default value for pos is 0 and len is String.length s - pos.