What is different in PCSCHEME's implementation of BGI?

<BODY bgcolor="#ffeee0" text="#000000" link="#9944EE" vlink="#FF0000" alink="#00ff00">  <A HREF="node11_mn.html" TARGET="_top"> <IMG WIDTH="37" HEIGHT="24" ALT="next" SRC="next.png"></A> <A HREF="node8_mn.html" TARGET="_top"> <IMG WIDTH="26" HEIGHT="24" ALT="up" SRC="up.png"></A> <A HREF="node9_mn.html" TARGET="_top"> <IMG WIDTH="63" HEIGHT="24" ALT="previous" SRC="prev.png"></A> <A NAME="tex2html262" HREF="node10_tf.html" TARGET="_top"> <IMG WIDTH="65" HEIGHT="24" ALT="contents" SRC="contents.png"></A> <A NAME="tex2html264" HREF="node10_id.html" TARGET="_top"> <IMG WIDTH="43" HEIGHT="24" ALT="index" SRC="index.png"></A> <BR> <B> Next:</B> <A HREF="node11_mn.html" TARGET="_top">Coordinate systems</A> <B> Up:</B> <A HREF="node8_mn.html" TARGET="_top">User's manual</A> <B> Previous:</B> <A HREF="node9_mn.html" TARGET="_top">What is BGI?</A>   <B> <A NAME="tex2html263" HREF="node10_tf.html" TARGET="_top">Contents</A></B>   <B> <A NAME="tex2html265" HREF="node10_id.html" TARGET="_top">Index</A></B> <BR> <BR> <P> Sorry, this requires a browser that supports frames! <BR> Try <A HREF="node10_ct.html">node10_ct.html</A> instead.</P> <HR>