[] type cm
// Represents a number with units of measure powered to the
// number's value (e.g "(S (S O))" has type Num)
type Num<[] 'M, [] 'N> =
| O_ of int * float<'N>
| S_ of int * Num<'M, 'N / 'M>
// Constructors that hide that simplify the creation
let O : Num<'M, 'M> = O_ (1, 0.0<_>)
let S n = match n with O_(i, _) | S_(i, _) -> S_(i + 1, n)
// Type-safe power function with units of measure
let pow (x:float<'M>) ((O_(i, _) | S_(i, _)):Num<'M, 'M 'N>) : float<'M 'N> =
// Unsafe hacky implementation, which is hidden
// from the user (for simplicity)
unbox ((float x) ** float i)
let res = pow 2.0 (S (S O))