fix docs generation with TeX Live 2017

[skip ci]

doc/crypt.tex

@@ -31,7 +31,6 @@
 \def\And{{\rm\ and\ }}
 \def\iff{\hspace{1em}\Longleftrightarrow\hspace{1em}}
 \def\implies{\Rightarrow}
-\def\undefined{{\rm \textit{undefined}}}
 \def\Proof{\vspace{1ex}\noindent {\bf Proof:}\hspace{1em}}
 \let\oldphi\phi
 \def\phi{\varphi}