struct A { mut: v int }  struct B { a A }  struct C { mut: b B } struct D { mut: c C }
mut b := B{} b = B{A{2}}
b.a.v = 1   // Error (field a immutable)
b.a = A{}   // Error (field a immutable)
===output===
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
mut:
	a A
}
cannot modify immutable field `a` (type `B`)
declare the field with `mut:`
struct B {
mut:
	a A
}