Restructured library documentation
diff --git a/Doc/lib/libmain.tex b/Doc/lib/libmain.tex
new file mode 100644
index 0000000..c730a03
--- /dev/null
+++ b/Doc/lib/libmain.tex
@@ -0,0 +1,6 @@
+\section{Built-in Module \sectcode{__main__}}
+
+\bimodindex{__main__}
+This module represents the (otherwise anonymous) scope in which the
+interpreter's main program executes --- commands read either from
+standard input or from a script file.