Recursive

: fib ( n-m ) dup [ 0 = ] [ 1 = ] bi or if; [ 1- fib ] sip [ 2 - fib ] do + ;


Iterative

2 elements previous result
: fib ( n-m )
  -1 !previous 1 !result
   1+ [ @result @previous over + !result !previous ] times @result ;
