property HasNext
  prefix "Iterator"
  prefix "List"
  prefix "Scanner"
  start -> start: *
  start -> invalid: I = iterator(*)
  start -> invalid: Scanner(I, *)  // For constructors, biabduction has "this" as first argument
  invalid -> valid: B = hasNext(i) if B != 0 // in SIL, "true" is encoded as "not 0"
  valid -> invalid: next(i)
  invalid -> error: next(i)