How to do I define a struct A which refer a struct B which refer A?

General Usage
1

For example:

mutable struct A
a::Int
b::Ref{B}
end

mutable struct B
b::Int
a::Ref{A}
end
2

Finally, I got a solution by Incomplete Initialization @Julia Manual.
Solution:

mutable struct A{T}
    a::Int
    b::Ref{T}
    A{T}(a::Int) where T = new{T}(a)
end

mutable struct B
    b::Int
    a::Ref{A{T}} where T
    B(b::Int)=new(b)
end

a =A{B}(1)
b = B(2)
a.b=Ref{B}()
a.b[]=b
a.b[].b == 2


b.a=Ref{A{B}}()
b.a[] = a
b.a[].a == 1