title: Linked list in julia
body: <p>can anybody help me to understand next things please: 
  <br>
  1) Why do we need to make a new abstract class of a future struct, while making linked list?
  <br>
  2) Why there is an argument T?
  <br>
  3) What does this operator do &lt;: (haven't found it in the book) ?
  <br>
  4) Can we write in the following way, instead of Example ?: 
  <br></p>
  
  <pre><code>type LinkedList 
      name = Ptr{Uint8}
      next :: LinkedList
  end
  </code></pre>
  
  <p>Example:</p>
  
  <pre><code>abstract type LinkedList{T} end
  
  mutable struct Nil{T} &lt;: LinkedList{T}
  end
  
  mutable struct Cons{T} &lt;: LinkedList{T}
     head::T
     tail::LinkedList{T}
  end
  </code></pre>
  
  <p>Thank you!</p>