Rank Function is Increasing

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

Let $\rho : \powerset S \to \Z$ be the rank function of $M$.

Let $A, B \subseteq S$ be subsets of $S$ such that $A \subseteq B$.

Then:
 * $\map \rho A \le \map \rho B$