Rising Factorial in terms of Falling Factorial of Negative

Theorem

 * $x^{\overline k} = \paren {-1}^k \paren {-x}^{\underline k}$

where:
 * $x^{\underline k}$ is the $k$ to the $x$ falling
 * $x^{\overline k}$ is the $k$ to the $x$ rising.