Fix truncation in PRF^sn and PRF^pk. closes #686

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

protocol/protocol.tex

@@ -190,9 +190,6 @@ a few differently-sized fields.}
 $\Leading{k}(x)$, where $k$ is an integer and $x$ is a bit sequence, returns
 the leading (initial) $k$ bits of its input.
 
-$\Trailing{k}(x)$, where $k$ is an integer and $x$ is a bit sequence, returns
-the trailing (final) $k$ bits of its input.
-
 \subsection{Cryptographic Functions}
 
 $\CRH$ is a collision-resistant hash function. In \Zcash, the $\SHAName$ function
@@ -226,7 +223,7 @@ independent.
         \bitbox{242}{256 bit $\SpendAuthorityPrivate$} &
         \bitbox{14}{0} &
         \bitbox{14}{1} &
-        \bitbox{242}{$\Trailing{254}(\CoinAddressRand)$} &
+        \bitbox{242}{$\Leading{254}(\CoinAddressRand)$} &
 \end{bytefield}
 \end{lrbox}
 
@@ -237,7 +234,7 @@ independent.
         \bitbox{14}{1} &
         \bitbox{14}{0} &
         \bitbox{14}{$i$} &
-        \bitbox{228}{$\Trailing{253}(\hSig)$}
+        \bitbox{228}{$\Leading{253}(\hSig)$}
 \end{bytefield}
 \end{lrbox}