Call Nat, PosInt and Rat types rather than sets, for consistency.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -857,8 +857,8 @@ one valid \nullifier, and so attempting to spend a \note twice would reveal the
 
 The notation $\bit$ means the type of bit values, i.e.\ $\setof{0, 1}$.
 
-The notation $\Nat$ means the set of nonnegative integers. $\PosInt$
-means the set of positive integers. $\Rat$ means the set of rationals.
+The notation $\Nat$ means the type of nonnegative integers. $\PosInt$
+means the type of positive integers. $\Rat$ means the type of rationals.
 
 The notation $x \typecolon T$ is used to specify that $x$ has type $T$.
 A cartesian product type is denoted by $S \times T$, and a function type