Sorry for my perhaps naive question.

When I compute 10^(p-2) using fast modular exponentiation as suggested by the challenge author I do not get the expected result of 700000005. My code for fast modular exponentiation is below (no spoiler here since it is a word for word rendering of the challenge author's own suggestion as he himself posted):

let rec modexp x y =
  if y=0. then 1.
  else
    let z=modexp x (floor (y/.2.)) in
        if (mod_float y 2. =0.) then  mod_p  (z ** 2.)
        else mod_p (x *. (z** 2.))
        
let mod_p a =
  mod_float ((mod_float a p) +. p) p
  
let p = 10. ** 9. +. 7.

Indeed, the reported result for modexp 10. (p-.2.) is 485926138.

Can someone point me in the right direction? Thank you!