struct Aaa { mut: v int }  struct Bbb { a Aaa }  struct Ccc { mut: b Bbb } struct Ddd { mut: c Ccc }
mut c := Ccc{} c.b = Bbb{}
c.b.a = Aaa{} // Error (field a immutable)
c.b.a.v = 1 // 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
}
