Cardinality Less One

Theorem
Let $S$ be a finite set.

Let:
 * $\card S = n + 1$

where $\card S$ is the cardinality of $S$.

Let $a \in S$.

Then:
 * $\card {S \setminus \set a} = n$

where $\setminus$ denotes set difference.

Proof
This follows as an immediate consequence of Set Equivalence Less One Element.