Sphere is Set Difference of Closed Ball with Open Ball

Theorem
Let $M = \struct{A, d}$ be a metric space or pseudometric space.

Let $a \in A$.

Let $\epsilon \in \R_{>0}$ be a strictly positive real number.

Let $\map {{B_\epsilon}^-} a$ denote the $\epsilon$-closed ball of $a$ in $M$.

Let $\map {B_\epsilon} a$ denote the $\epsilon$-open ball of $a$ in $M$.

Let $\map {S_\epsilon} a$ denote the $\epsilon$-sphere of $a$ in $M$.

Then:
 * $\map {S_\epsilon} a = \map {{B_\epsilon}^-} a \setminus \map {B_\epsilon} a$