new command '`' to show discoveries for one class (trunk only)

Use the grave accent (back tick) character as the keystroke for a
new command which prompts for an object class and then shows a subset of
the discovered objects list covering just the selected class.  Similar
to the 'I' variant of 'i' for viewing inventory, and mainly useful once
the '\' discoveries list has grown long.

doc/Guidebook.tex

@@ -33,7 +33,7 @@
 \begin{document}
 %
 % input file: guidebook.mn
-% $Revision: 1.115 $ $Date: 2011/04/24 21:12:06 $
+% $Revision: 1.116 $ $Date: 2011/04/25 03:29:41 $
 %
 %.ds h0 "
 %.ds h1 %.ds h2 \%
@@ -46,7 +46,7 @@
 %.au
 \author{Eric S. Raymond\\
 (Extensively edited and expanded for 3.5)}
-\date{April 17, 2011}
+\date{September 14, 2011}
 
 \maketitle
 
@@ -491,7 +491,7 @@ An amulet or a spider web.
 \item[\tb{*}]
 A gem or rock (possibly valuable, possibly worthless).
 %.lp
-\item[\tb{`}]
+\item[\tb{\`}]
 A boulder or statue.
 %.lp
 \item[\tb{0}]
@@ -957,6 +957,9 @@ ordering.)
 \item[\tb{$\backslash$}]
 Show what types of objects have been discovered.
 %.lp
+\item[\tb{\`}]
+Show discovered types for one class of objects.
+%.lp
 \item[\tb{!}]
 Escape to a shell.
 %.lp