\usepackage{xcolor}

\usepackage{fontspec}

\usepackage[sc]{mathpazo}

\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}

\newcommand{\dn}{\stackrel{\mbox{\scriptsize def}}{=}}%

\definecolor{codegray}{gray}{0.9}

\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	

\makeatletter

\def\fnote{\gdef\@thefnmark{}\@footnotetext}

\makeatother

\newcommand{\HEADER}{{\bf Please submit your solutions to the email address 

7ccsmsen at gmail dot com. Please submit only ASCII text or PDFs. Every solution 

should be preceeded by the corresponding question. Solutions will only be accepted until 

30th December!}\bigskip}