[ # 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