\usepackage{xcolor}
\usepackage{fontspec}
\usepackage[sc]{mathpazo}
\usepackage{fontspec}
\setmainfont[Ligatures=TeX]{Palatino Linotype}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage{menukeys}
\definecolor{darkblue}{rgb}{0,0,0.6}
\usepackage[colorlinks=true,urlcolor=darkblue,linkcolor=darkblue]{hyperref}

%%% for trees
%% http://anorien.csc.warwick.ac.uk/mirrors/CTAN/graphics/pgf/contrib/forest/forest.pdf

\newcommand\grid[1]{%
\begin{tikzpicture}[baseline=(char.base)]
  \path[use as bounding box]
    (0,0) rectangle (1em,1em);
  \draw[red!50, fill=red!20]
    (0,0) rectangle (1em,1em);
  \node[inner sep=1pt,anchor=base west]
    (char) at (0em,\gridraiseamount) {#1};
\end{tikzpicture}}
\newcommand\gridraiseamount{0.12em}

\makeatletter
\newcommand\Grid[1]{%
  \@tfor\z:=#1\do{\grid{\z}}}
\makeatother	

\newcommand\Vspace[1][.3em]{%
  \mbox{\kern.06em\vrule height.3ex}%
  \vbox{\hrule width#1}%
  \hbox{\vrule height.3ex}}

\def\VS{\Vspace[0.6em]}


\newcommand{\dn}{\stackrel{\mbox{\scriptsize def}}{=}}
\newcommand{\defn}[1]{\textit{\textbf{#1}}}
\newcommand{\dq}[1]{\mbox{\tt{"}}#1\mbox{\tt{"}}}

\definecolor{codegray}{gray}{0.9}

\newcommand{\HEADER}{{\bf Please submit your solutions via email. Please submit 
only ASCII text or PDFs. Every solution should be preceeded by the corresponding 
question, like:

\begin{center}
\begin{tabular}{ll}
Q$n$: & \ldots a difficult question from me\ldots\\
A:    & \ldots an answer from you \ldots\\
Q$n+1$ & \ldots another difficult question\ldots\\
A:     & \ldots another brilliant answer from you\ldots
\end{tabular}
\end{center}

\noindent Solutions will only be accepted until 30th December!}\bigskip}