struct Aaa { mut: v int }  struct Bbb { a Aaa }  struct Ccc { mut: b Bbb } struct Ddd { mut: c Ccc }
mut b := Bbb{} b = Bbb{Aaa{2}}
b.a.v = 1   // Error (field a immutable)
b.a = Aaa{}   // Error (field a immutable)
===output===
cannot modify immutable field `a` (type `Bbb`)
declare the field with `mut:`
struct Bbb {
mut:
	a Aaa
}
cannot modify immutable field `a` (type `Bbb`)
declare the field with `mut:`
struct Bbb {
mut:
	a Aaa
}