I would like to center the Available Expression, Very Busy Expression... Live Variables cells. The following code aligns the quoted cells only on the bottom of the cell itself: is there a way to center vertically even those cells? Thanks in advance.
\usepackage{bbm}
\usepackage[italian]{babel}
\usepackage[italian]{varioref}
\usepackage{rotating}
\usepackage{pgf,tikz}
\usepackage{etex}
\usepackage{tikz-cd}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[utf8]{inputenx}
\usepackage{braket}
\usepackage[a4paper]{geometry}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathabx}
\usepackage{examples}
\usepackage{multirow}
\newcommand{\spheading}[2][10em]{% \spheading[<width>]{<stuff>}\rotatebox{90}{\parbox{#1}{\raggedright #2}}}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}
......
\begin{tabular}{c|M{20mm} M{30mm}}
& forward ($flow$) & backward ($flow^R$)\\
\midrule
\spheading[4em]{must ($\cap$)} & Available Expression & Very Busy Expression\\
\spheading[4em]{may ($\cap$)} & Reaching Definition & Live Variables\\
\bottomrule
\end{tabular}


$flow$, LaTeX typesets this as the quantitiesf,l,oandwall "multiplied together". You should use$\mathit{flow}$and$\mathit{flow}^R$instead. Also, one of your\caps should be a\cup. – John Wickerson May 22 '13 at 14:42.texfile, starting with\documentclass. Otherwise people trying to reproduce your problem have to guess which packages (e.g. booktabs) you've loaded. – John Wickerson May 22 '13 at 14:55