Missing } inserted (xypic code)

Question

Good afternoon,

Unfortunately I'm getting a "! Missing } inserted.

<inserted text> 
                }
l.23 **\xymatrix{$\CIRCLE$ $\CIRCLE$ & $\CIRCLE$ }
                                                  **
? "

What's wrong with my code, as far as I can tell, all my braces are matched :\

Thanks for all suggestions,

---

\documentclass[a4paper,11pt]{article}
\usepackage[english]{babel}
\usepackage[pdftex]{graphicx}
\usepackage{tikz}
\usepackage{wasysym}
\usepackage[all]{xy}
\usepackage{amsfonts}

\title{Testcase}

\author{Alec Taylor}

\date{August 23, 2011}

\begin{document}

\xymatrix{$\CIRCLE$ $\CIRCLE$ & $\CIRCLE$ } % <-- Causes the error

\end{document}

Answer 1

\xymatrix{...} implies math mode, so you don't have to (and, in fact, mustn't) use $ in the cells.

Your code works if you just remove the $:

\xymatrix{\CIRCLE \CIRCLE & \CIRCLE }