WIP for abstract PRF definitions.

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

protocol/protocol.tex

@@ -819,9 +819,16 @@ indicates the output length in bits. It is used in \crossref{hsig} and
 
 $\PRF{x}{}$ is a \pseudoRandomFunction seeded by $x$. \changed{Four} \emph{independent}
 $\PRF{x}{}$ are needed in our protocol:
-$\PRFaddr{} \typecolon $, $\PRFnf{x}$, $\PRFpk{x}$\changed{,
-and $\PRFrho{x}$}. These are used in \crossref{circuit}, and instantiated in
-\crossref{prfs}.
+\begin{equation*}
+\begin{aligned}
+&\PRFaddr{} &\typecolon \\
+&\PRFnf{}   &\typecolon \\
+&\PRFpk{}   &\typecolon \\
+&\PRFrho{}  &\typecolon.
+\end{aligned}
+\end{equation*}
+
+These are used in \crossref{circuit}, and instantiated in \crossref{concreteprfs}.
 
 \securityrequirement{
 In addition to being \pseudoRandomFunctions, it is required that $\PRFnf{x}$