The Fibonacci sequence using Forth language

I used GNU Forth (https://www.gnu.org/software/gforth).

Forth is a stack based language. All functions are called words.

• 2dup word duplicates n and n-1 elements and put it on the top of the stack
• + word takes n and n-1 elements from the stack and sum them. The result goes on top of the stack.

: fib ( x1 x2 -- x1 x2 x3 )
    2dup
    +
;

The fib word sums the two last elements and put the result on the top of the stack, which is the Fibonacci sequence.

First number is the occurrence of the sequence and the second its result.