+ char * new_string = try_malloc(old_size + add_nl + new_size + 1, __func__);
+ memcpy(new_string, *string, old_size);
+ char * pattern = add_nl ? "\n%s" : "%s";
+ int test = sprintf(new_string + old_size, pattern, append);
+ exit_trouble(test < 0, __func__, "sprintf");
+ free(*string);
+ *string = new_string;