Referring to another member variable

```nim
class A:
  var a: int
  # referring to member variable `a` with {.initial.}
  var b {.initial.} = self.a
```
is roughly the same as
```nim
class A:
  var a: int
  var b: int
  proc `new`(a) =
    self.a = a
    self.b = self.a
```