List specification (cont.)
npublic Student get(int i)
n The element at the specified position.
n require:0<=i && i<this.size()
npublic void append (Student s)
n Append the specified Student to the end of this List.
n require:
n   s!=null
n ensure:
n    this.size()==old.size+1
n    this.get(this.size()-1) == s
nAnything not mentioned does not change!