Unity Divides All Elements/Proof 1

Proof
The element $1_D$ is the unity of $\struct {D, +, \circ}$, and so:
 * $1_D \in D: x = 1_D \circ x$

Similarly, from Product of Ring Negatives:


 * $-1_D \in D: x = \paren {-1_D} \circ \paren {-x}$

The result follows from the definition of divisor.