/ Euclid gcd
/ Last 2 numbers on the stack are arguments
/ Result is passed through the stack too
F {
    / Parse arguments into variables
    &a=a
    &b=b

    / If a == b then exit with it
    $ab?#;

    b[-] &c=c   / a - b
    &f^+=f c (  / if a - b > 0 (or a > b)
        $c $b
    ^=f#) f (   / else
        $b a[-] $a
    #)

    / Call ourselves but
    / with simpler case
    @F
}

/ gcd(4, 5)
^++++ ^+++++ @F