fibo := {
  takes '[n].
  cache := here cache.
  { cache slot? (n ordinal). } ifFalse {
    cache slot (n ordinal) =
      if { n <= 1. } then {
        n.
      } else {
        fibo (n - 1) + fibo (n - 2).
      }.
  }.
  cache slot (n ordinal).
} tap {
  ;; Attach the cache to the method object itself.
  #'self cache := Object clone.
}.