:: three ( lambda -- )
  "three" .
  B lambda [ random-int swap keep ] dup bi
  "three-ifs" .
  2dup [ 0 <= ] bi@ or [
    2dup [ 0 <= ] bi@ and [
      2drop lambda three
    ] [
      random-real lambda keep
      2nip 0 <= [ lambda three ] when
    ] if
  ] [
    2drop
  ] if
;