bool ch_is_digit ( char c );
int ch_to_digit ( char ch );
void i4vec_print ( int n, int a[], string title );
int luhn_check_digit_calculate ( string s );
int luhn_checksum ( string s );
bool luhn_is_valid ( string s );
int s_digits_count ( string s );
int *s_to_digits ( string s, int n );
void timestamp ( );