original.name="Template_Valid_17"
======
>>> main.whiley
type LinkedList<T> is null | { LinkedList<T> next, T data }

property length<T>(LinkedList<T> list, int n) -> (bool r):
    return (list is null && n == 0) ||
           (!(list is null) && length<T>(list.next,n-1))

function recursive<T>(LinkedList<T> l) -> (int len)
ensures length<T>(l,len):
    //
    if l is null:
        return 0
    else:
        return 1 + recursive<T>(l.next)


public export method test():
    LinkedList<int> l1 = null
    LinkedList<int> l2 = { next: l1, data: 1 }
    LinkedList<int> l3 = { next: l2, data: 2 }
    //
    assert length<int>(l1,0)
    assert length<int>(l2,1)
    assert length<int>(l3,2)
    
---