How can the word “is” express class inclusion in formal logic?

In formal logic, the natural-language verb "to be" is considered multiply ambiguous. When it is used to express class inclusion—such as in the statement "The whale is a mammal"—it is not treated as a simple predication or identity statement.12

Instead, logicians represent this relationship using the universal quantifier and the material conditional. The logical form is expressed as:

$$(\forall x)(W(x)\supset M(x))$$

This formula translates to: "For all x, if x is a whale, then x is a mammal." In this context, the "is" of class inclusion signifies that the set of objects satisfying the first condition (being a whale) is included within the set of objects satisfying the second condition (being a mammal). This is distinct from set theory, where class inclusion is formally defined as the relation where every member of one class is also a member of another, written as $x\subseteq y$.123