struct A { mut: v int }  struct B { a A }  struct C { mut: b B } struct D { mut: c C } 
mut c := C{} c.b = B{}
c.b.a = A{} // Error (field a immutable)
c.b.a.v = 1 // 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
}