see "42 %! 2017 = " + multInv(42, 2017) + nl
func multInv a,b
b0 = b
x0 = 0
multInv = 1
if b = 1 return 0 ok
while a > 1
q = floor(a / b)
t = b
b = a % b
a = t
t = x0
x0 = multInv - q * x0
multInv = t
end
if multInv < 0 multInv = multInv + b0 ok
return multInv