\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 theemail address 7ccsmsen at gmail dot com. Please submit onlyASCII text or PDFs. Every solution should be preceded by thecorresponding 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 until30th December!}\bigskip}