Restriction of Well-Founded Relation is Well-Founded

Theorem
Let $\struct {S, \RR}$ be a relational structure.

Let $\RR$ be a well-founded relation on $S$.

Let $T$ be a subset or subclass of $S$.

Let $\RR'$ be the restriction of $\RR$ to $T$.

Then $\preceq'$ is a well-founded relation on $T$.

Proof
Let $A$ be a non-empty subset of $T$.

By Subset Relation is Transitive, $A$ is a non-empty subset of $S$.

Since $\RR$ is a well-founded relation on $S$, $A$ has a minimal element $m$ under $\RR$.

Let $x \in A$.

Let $x \mathrel {\RR'} m$.

By the definition of restriction:
 * $x \mathrel \RR m$

Thus by the definition of minimal element:
 * $x = m$

As $x$ is an arbitrary element of $A$, this holds for all $x \in A$.

Hence by definition $m$ is a minimal element of $A$ under $\RR'$.

As $A$ is an arbitrary non-empty subset of $T$, this holds for all non-empty subsets of $T$.

Hence $\RR'$ is a well-founded relation on $T$.