1) defining a quasi-recursive function combined with a simple Ω-combinator:
{def fibo {lambda {:n}
 {{{lambda {:f} {:f :f}}
  {lambda {:f :n :a :b}
   {if {< :n 0}
    then the number must be positive!
    else {if {<  :n 1}
    then :a
    else {:f :f {- :n 1} {+ :a :b} :a}}}}} :n 1 0}}}
-> fibo

2) testing:
{fibo -1} -> the number must be positive!
{fibo 0} -> 1
{fibo 8} -> 34
{fibo 1000} -> 7.0330367711422765e+208
{S.map fibo {S.serie 1 20}}
-> 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765 10946

We could also avoid any name and write an IIFE

{{lambda {:n}
 {{{lambda {:f} {:f :f}}
  {lambda {:f :n :a :b}
   {if {< :n 0}
    then the number must be positive!
    else {if {<  :n 1}
    then :a
    else {:f :f {- :n 1} {+ :a :b} :a}}}}} :n 1 0}}
 8}
-> 34