const unsigned char *begin() const { return data ? data+1 : 0; }
const unsigned char *end() const { return data ? data+1+data[0] : 0; }
unsigned stride() const;
const unsigned char *begin() const { return data ? data+1 : 0; }
const unsigned char *end() const { return data ? data+1+data[0] : 0; }
unsigned stride() const;