* function: cstring_t->expand to add one character to the struct
* function: cstring_t->expand_arr to add a string to the struct
* function: cstring_t->strip to remove a substring
* function: cstring_t->expand to add one character to the struct
* function: cstring_t->expand_arr to add a string to the struct
* function: cstring_t->strip to remove a substring