# Calling singular.invariant_ring

I am trying to use Singular through Sagemath, to compute an the invariant subring of a finite group. Here is a minimal example where the error I get occurs:

```
singular.lib('finvar.lib')
R = singular.ring(3, '(x0,x1)', 'dp')
g = singular.matrix(2,2,'1,0,1,1')
singular.invariant_ring(g)
```

I saved this code as `test.sage`

and ran `sage test.sage`

(Debian 9, sagemath installed using `apt`

, with an existing installation of Singular), and got the error (I quote only the last few lines)

```
File "/usr/lib/python2.7/dist-packages/sage/interfaces/singular.py", line 653, in eval
raise SingularError('Singular error:\n%s'%s)
TypeError: Singular error:
? assign not impl.
? error occurred in or before STDIN line 16: ` return(P,S);`
```

However the corresponding (or at least I believe so) Singular code

```
LIB"finvar.lib";
ring R = 3,(x0,x1),dp;
matrix g1[2][2]= 1,0,1,1;
matrix P, S=invariant_ring(g1);
```

runs properly. What is the problem? Instead of the last line in my sage code, I tried

```
P, S = singular.invariant_ring(g)
```

and

```
[P, S] = singular.invariant_ring(g)
```

but this did not help. I get the same error when I tried the code at the Sage Cell Server.