Some equation for a hasp passwords
 

There is a next equation for the nibbles of a hasp passwords:
a4 = (a1^9 - a0^1 -((a1^9 + a2 + ((a3^9 + a2) >= 15)) <= 15 )) %8
(see forum.ru-board.com)
for pass 1177:0a28
a0 = 1 , a1 = 1 , a2 = 7 , a3 = 7
a4 = 0 , a5 = a , a6 = 2 , a7 = 8
what it means?

we have 8 var., but have only one equation.

i can't imagine the solution without bruteforing.

have any idea??

It was found by analysis about 500 passwords