Iteratively grows two refs while the larger is below 10^(n-1),
counting iterations:
let euler25 n =
let a = ref 1 in
let b = ref 1 in
let i = ref 2 in
let target = ref 1 in
for _ = 1 to n - 1 do target := !target * 10 done;
while !b < !target do
let c = !a + !b in
a := !b;
b := c;
i := !i + 1
done;
!i
euler25 12 = 55 (F(55) = 139_583_862_445, 12 digits)
Real PE25 asks for 1000 digits (answer 4782); 12 keeps within
safe-int while exercising the identical algorithm.
114 baseline programs total — 200 iterations landed.
