interface IntList {
  var list: seq[int]

  constructor Empty()
    ensures list = []

  constructor SingletonTwo()
    ensures list = [2]

  constructor OneTwo()
    ensures list = [1 2]

  constructor Singleton(p: int)
    ensures list = [p]

  constructor TwoConsecutive(p: int)
    ensures list = [p] + [p+1]

  constructor Double(p: int, q: int)
    ensures list = [p] + [q]

  constructor Sum(p: int, q: int)
    ensures list = [p + q]
}

datamodel IntList {
  var root: IntNode

  frame
    root

  invariant
    root = null  ==> |list| = 0
    root != null ==> (|list| = |root.succ| + 1 && 
                      list[0] = root.data && 
                      (forall i :: i in 1 ... |root.succ| ==> (root.succ[i-1] != null && list[i] = root.succ[i-1].data)))
}

interface IntNode {
  var succ: seq[IntNode]
  var data: int

  constructor Zero()
    ensures data = 0
    ensures succ = []

  constructor OneTwo()
    ensures data = 1
    ensures |succ| = 1 && succ[0] != null && succ[0].data = 2

  constructor Init(p: int)
    ensures data = p

  constructor InitInc(p: int)
    ensures data = p + 1

    
  invariant
    !(null in succ)
}

datamodel IntNode {
  var next: IntNode

  frame 
    next

  invariant
    next = null  ==> |succ| = 0
    next != null ==> (succ = [next] + next.succ)
}