Sandwich Principle for Minimally Closed Class

Theorem
Let $N$ be a class which is closed under a progressing mapping $g$.

Let $b$ be an element of $N$ such that $N$ is minimally closed under $g$ with respect to $b$.

Then for all $x, y \in N$:


 * $x \subseteq y \subseteq \map g x \implies x = y \lor y = \map g x$

Proof
From Minimally Closed Class induces Nest, we have that $N$ is a nest in which:
 * $\forall x, y \in N: \map g x \subseteq y \lor y \subseteq x$

Thus the Sandwich Principle applies directly.