╭    x!     y      x-y      x!     k      x-k ╮
... = n │ ──────── t  (1-t)    - ──────── t  (1-t)    │  , with x=n-1, y=k-1
        ╰ y!(x-y)!               k!(x-k)!             ╯                     
... = n (B           (t) - B       (t))                                     
          (n-1),(k-1)       (n-1),k