# SAT Solver output

I need to solve a SAT problem. I searched and found the manual here, but can not figure out the output obtained:

```
sage: from sage.sat.solvers import CryptoMiniSat # optional - cryptominisat
sage: cms = CryptoMiniSat() # optional - cryptominisat
sage: cms.add_clause((1,2,-3)) # optional - cryptominisat
sage: cms() # optional - cryptominisat
(None, True, True, False)
```

There are 3 variables, then what does that `None`

mean?