clarify sqrt; closes #394

Author: rzach

content/sets-functions-relations/functions/function-basics.tex

@@ -64,9 +64,9 @@
 \begin{ex}
 Multiplication is a function because it pairs each input---each pair
 of natural numbers---with a single output: $\times \colon \Nat^2 \to
-\Nat$. By contrast, the square root operation applied to the domain
-$\Nat$ is not functional, since each positive integer $n$ has two
-square roots: $\sqrt{n}$ and $-\sqrt{n}$. We can make it functional by
+\Nat$. By contrast, mapping a natural number~$n$ to a real~$x$ such
+that $x^2 = n$ is not functional, since each positive integer~$n$ has two
+square roots: $\sqrt{n}$ and~$-\sqrt{n}$. We can make it functional by
 only returning the positive square root: $\sqrt{\phantom{X}} \colon
 \Nat \to \Real$. 
 \end{ex}