²⁰This special value used to be string_undefined, but its name carried less semantics. Empty comments could be defined either as NULL, or the null length string, ””, or as the Newgen special value, string_undefined. The NULL string was not chosen because it is not specific enough and because it is not compatible with the UNIX string library. The null length string is compatible with the string library, but sharing would have to be carefully considered. To avoid many allocations and desallocations of one byte areas, and many storage related bugs, the Newgen solution was chosen, although it is not compatible with the UNIX library and although Newgen does not provide such a library. Objects of type string are not 100 % equivalent to objects of type char * and guards for string_undefined must be added.