old_contents: 
  
  x = []
  
  x.append("hallo") # FIXME: how to handle/test internal/unreachable clases & methods # type 
new_contents: a = list()
  a.append(1)
  a.pop() ## type int
  
  b = list()
  b.append(3.14)
  b.pop() ## type float
  
subject: Test for builtins.