Move the definition of ⊥ to before its first use.

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

protocol/protocol.tex

@@ -2713,6 +2713,9 @@ $\setof{x \typecolon T \suchthat e_x \not\in V}$ and range $U$.
 
 $\powerset{T}$ means the powerset of $T$.
 
+$\bot$ is a distinguished value used to indicate unavailable information,
+or a failed decryption or validity check.
+
 $\typeexp{T}{\ell}$, where $T$ is a type and $\ell$ is an integer,
 means the type of sequences of length $\ell$ with elements in $T$. For example,
 $\bitseq{\ell}$ means the set of sequences of $\ell$ bits, and
@@ -2854,9 +2857,6 @@ $\ceiling{x}$ means the smallest integer $\geq x$.
 $\bitlength(x)$, for $x \typecolon \Nat$, means the smallest integer
 $\ell$ such that $2^\ell > x$.
 
-The symbol $\bot$ is used to indicate unavailable information, or a failed
-decryption or validity check.
-
 \introlist
 The following integer constants will be instantiated in \crossref{constants}:
 \begin{formulae}
@@ -14026,6 +14026,7 @@ Peter Newell's illustration of the Jubjub bird, from \cite{Carroll1902}.
     \item Describe the recommended way to encode a \Sapling\nufive{ or unified} \paymentAddress
           as a QR code.
 } %sapling
+    \item Move the definition of $\bot$ to before its first use.
     \item Add a definition for the \S{} symbol in \crossref{introduction}, before its first use.
     \item Remove specification of \memo contents, which will be in \cite{ZIP-302}.
     \item Remove support for building the \Sprout-only specification (\texttt{sprout.pdf}).