Element of Matroid Base and Circuit has Substitute/Lemma 1

Theorem
Let $M = \struct {S, \mathscr I}$ be a matroid.

Let $B \subseteq S$ be a base of $M$.

Let $C \subseteq S$ be a circuit of $M$.

Let $x \in B \cap C$.

Then:
 * $C \setminus \set x$ is an independent proper subset of $C$

Proof
By definition of circuit:
 * $C$ is a minimal dependent subset

From Singleton of Element is Subset:
 * $\set x \subseteq C$ and $\set x \subseteq B$

From matroid axiom $(\text I 2)$:
 * $\set x \in \mathscr I$

Because $C \notin \mathscr I$:
 * $\set x \ne C$

Hence:
 * $\set x \subsetneq C$

From Set Difference with Proper Subset is Proper Subset:
 * $C \setminus \set x$ is a proper subset of $C$

As $C$ is a minimal dependent subset:
 * $C \setminus \set x \in \mathscr I$