[               # todo: n(<2) -- 1 and break 2 levels
  d -           # 0
  1 +           # 1
  q
] s1

[               # todo: n(>-1) -- F(n)
  d 0=1         # n(!=0)
  d 1=1         # n(!in {0,1})
  2 - d 1 +     # (n-2) (n-1)
  lF x          # (n-2) F(n-1)
  r             # F(n-1) (n-2)
  lF x          # F(n-1)+F(n-2)
  +
] sF

33 lF x f