I have a function of 3 variables F = F(a,b,c),

3 functions of 2 variables A(m,n), B(m,n) and C(m,n)

How do we input this in cadabra?

```
f(m,n) = F(A(m,n),B(m,n),C(m,n))
```

I also want to calculate the chain rule (partial derivative) and calculate the 2nd partial derivative.