# Pollard method for discrete logarithms (sage code)

I'm trying to run the following Sage code from Introduction to Cryptography with Open-Source Software:

```
sage: a,y,n = 7,12,71
sage: R.<X> = Zmod(n)[]; R.<Y> = Zmod(n)[]
sage: def psi(X):
....: x,r,s=X[0],X[1],X[2]
....: if x%3==0:
....: return [(x^2)%n,(2*r)%(n-1),(2*s)%(n-1)]
....: if x%3==1:
....: return [(a*x)%n,(r+1)%(n-1),s]
....: if x%3==2:
....: return [(y*x)%n,r,(s+1)%(n-1)]
sage: for i in range(1,11):
....: X = psi(X); Y = psi(psi(Y))
....: print(i,X,Y,X[0]==Y[0])
....:
```

And I get the following error:

```
ArithmeticError: reduction modulo 3 not defined.
```

Any idea? Thank you.