Denominator must be non-zero

Author: ktgw0316

src/content/3.10/ends-and-coends.tex

@@ -345,7 +345,7 @@ \section{Coends}
 set by picking one representative from each class. A classic example is
 the definition of rational numbers as pairs of whole numbers with the
 following equivalence relation:
-\[(a, b) \sim (c, d)\ \text{iff}\ a * d = b * c\]
+\[(a, b) \sim (c, d)\ \text{iff}\ a * d = b * c,\ b \neq 0,\ d \neq 0\]
 It's easy to check that this is an equivalence relation. A pair
 $(a, b)$ is interpreted as a fraction $\frac{a}{b}$, and
 fractions whose numerator and denominator have a common divisor are identified. A rational number