Class BigNum {
      a=stack
      Function Digits {
            =len(.a)*14-(14-len(str$(stackitem(.a,len(.a)) ,"")))
      }
      Operator "+" (n) {
            \\ we get a copy, but .a is pointer
             \\ we make a copy, and get a new pointer
            .a<=stack(.a)
            acc=0
            carry=0
            const d=100000000000000@
                  k=min.data(Len(.a), len(n.a))
                  i=each(.a, 1,k )
                  j=each(n.a, 1,k)
                  while  i, j {
                        acc=stackitem(i)+stackitem(j)+carry
                        carry= acc div d
                        return .a, i^+1:=acc mod d
                  }
                  if len(.a)<len(n.a) Then  {
                        i=each(n.a, k+1, -1)
                        while i {
                              acc=stackitem(i)+carry
                              carry= acc div d
                              stack .a  {data acc mod d}
                        }
                  } ELse.if len(.a)>len(n.a) Then  {
                        i=each(.a, k+1, -1)
                        while i {
                              acc=stackitem(i)+carry
                              carry= acc div d
                              Return .a, i^+1:=acc mod d
                              if carry else exit
                        }
                  }
                  if carry then stack .a { data carry}
      }
      Function tostring$ {
            if len(.a)=0 then ="0" : Exit
            if len(.a)=1 then =str$(Stackitem(.a),"") : Exit
            document buf$=str$(Stackitem(.a, len(.a)),"")
            for i=len(.a)-1 to  1 {
                  Stack .a {
                        buf$=str$(StackItem(i), "00000000000000")
                  }
            }
            =buf$
      }
      class:
      Module BigNum (s$) {
            s$=filter$(s$,"+-.,")
            if s$<>""  Then {
                  repeat {
                        If len(s$)<14 then Stack .a { Data  val(s$) }: Exit
                        Stack .a { Data  val(Right$(s$, 14)) }
                        S$=Left$(S$, len(S$)-14)
                  } Until S$=""
            }
      }
}

Inventory K=0:=BigNum("0"),1:=BigNum("1")
fib=Lambda K (x as decimal)-> {
      If Exist(K, x) Then =Eval(K) :Exit
      Ret=If(x>1->Lambda(x-1)+Lambda(x-2), bignum(str$(x,"")))
      Append K, x:=Ret
      =Ret
}
\\ Using this to handle form  refresh by code
Set Fast!
For i=1 to 4000 {
      N=Fib(i)
      Print i
      Print N.tostring$()
      Refresh
}