In words, we would say that an animal is in the set \(A \leftrightarrow B\) fs that animal is *both* (either not duck-billed or a mammal) *and* (not a mammal or duck-billed).

Note that if it is duck-billed then it must be a mammal;

if it is a mammal then it must be duck-billed.

The relation \( A \leftrightarrow B\) can also be expressed as:

**A** if and only if **B**;

**B** if and only if **A**.