/ Takes an index in the fibonacci sequence on the stack
/ Returns a number of the fibonacci sequence with a given index
F {
    / Let us define some local variables
    &a &b &c

    / If index is zero or one then we just return the number
    ?c #; $c+ = c
    ?c #;

    / Otherwise we get previous two numbers of the sequence
    / and return their sum
    = c $c $c
    - @F = a
    -- @F = b
    $a $b*
}

^+++++++++++++++++++++++=t$t
$t@F