for x0, x1, x2, x3, a, b, c being Real
for f being Function of REAL,REAL st ( for x being Real holds f . x = ((a * (x ^2)) + (b * x)) + c ) & x0,x1,x2,x3 are_mutually_distinct holds
[!f,x0,x1,x2,x3!] = 0