Hint:
• Enter a number in "nbr" → "bns_nbr" will be calculated with BNS_encode
• Enter a BNS string in "bns_nbr" → "nbr" will be calculated with BNS_decode
• "verify" shows the original value after back conversion
• Extreme values: Largest BNS number (7y999...) → 9 (plus infinity), smallest → 0 (minus infinity)