Recursive attribute sets can reference values declared earlier in the same set. 
keyword: `rec`
```nix
rec {
  one = 1;
  two = one + 1;
}
```

Ordering does not matter:
```nix
nix-repl> rec { two = one + 1; one = 1; }
{
  one = 1;
  two = 2;
}
```