:: (unfold) 
    ( seed 
      p:   ( seed -- ? )
      f:   ( seed -- o )
      g:   ( seed -- seed' )
      -- )
  seed p call [ 
    seed f call , seed g call p f g (unfold)
  ] unless ; inline recursive

: unfold  ( seed p f g -- seq )
  [ (unfold) ] { } make ; inline

! fib
{ 0 1 } 
[ first 30 > ]
[ first ]
[ first2 swap over + 2array ]  
unfold

! fac
100000 [ 1 < ] [ ] [ 1 - ] unfold 1 [ * ] reduce