)abbrev package TESTP TestPackage
Z ==> Integer
TestPackage : with
    fib : Z -> Z
  == add
    fib x ==
      x <= 0 => error "argument outside of range"
      f : Reference((Z,Z,Z) -> Z) := ref((n, v1, v2) +-> 0)
      f() := (n, v1, v2) +-> if n<2 then v2 else f()(n-1,v2,v1+v2)
      f()(x,1,1)