Returns a raw text string that contains a latex representation of
        the belief state as an attribute-value matrix.  This requires:
            \usepackage{avm}

def to_latex(self, number=0):
        """ Returns a raw text string that contains a latex representation of
        the belief state as an attribute-value matrix.  This requires:
            \usepackage{avm}
        """ 
        latex = r"""\avmfont{\sc}
\avmoptions{sorted,active}
\avmvalfont{\rm}"""
        latex += "\n\nb_%i = \\begin{avm} \n " % number
        latex += DictCell.to_latex(self)
        latex += "\n\\end{avm}\n"
        return latex