[plt-scheme] SLaTeX question

When I process the following input file with pdf-slatex:

\documentclass{article}
\usepackage[pdftex,usenames,dvipsnames]{color}
\usepackage{slatex}
\def\keywordfont#1{\textcolor{red}{#1}}
\def\variablefont#1{\textcolor{blue}{#1}}
\def\constantfont#1{\textcolor{Black}{#1}}
\def\schemecodehook{\color{green}}
\pagestyle{plain}
\begin{document}
\begin{schemedisplay}
(+ (* 6 4) (- 3 2))
\end{schemedisplay}
\end{document}

the parentheses are in green, the numbers are in black, the + and * are 
in blue. But the - is in green. Why? Thanks. --PR

(PS My twelve-year-old noticed this while being the audience for a 
practice lecture.)