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