How to call a function in smart contract by using tester.py

```
/* contract source code */
contract Hello {
   uint counter = 0;
   function count(uint food) public {
      counter += food;
   }
}

/* python source code */
c.tx(sender=k0, to=contract.count.address, data=3) 
```

How to call a function in smart contract by using tester.py ? There is a tutorial for tester module. but it is not detailed. Of course, above tx call does not work.
