\doc{Name classes}
\ref{class}
\ref{class-name}
\ref{class-constant}
\ref{class-function}

A \def{class} is an expression that, when evaluated, produces a list of
\slink{name}{name}s of facts.

\head{2}{\dlink{../symbols/syntax}{Syntax}}
\begin{verbatim}
\sd{class}          ::= \s{class-name} | \s{class-constant}
                        | \s{class-function} \qf{(} \slink{names}{names} \qf{)}
                        | \f{contains-operator} \qf{(} \slink{../logic/operator}{operator} \qf{)}
                        | \f{contains-variable} \qf{(} \slink{../logic/variable}{variable} \qf{)}
                        | \f{copy} \qf{(} \s{class-name} \qf{)}
\sd{class-name}     ::= \f{$} \slink{../symbols/symbols}{simpleId}
\sd{class-constant} ::= \f{deduction-rules} | \f{formulas} | \f{induction-rules}   
                       | \f{operator-theories} | \f{rewrite-rules} | \f{active} | \f{passive} 
                       | \f{immune} | \f{nonimmune} | \f{ancestor-immune}
\sd{class-function} ::= \f{ancestors} | \f{proper-ancestors} | \f{descendants}
                       | \f{proper-descendants} | \f{eval}
\end{verbatim}


\head{2}{Examples}
\begin{verbatim}
$facts
contains-operator(+)
eval(* ~ $old)
\end{verbatim}

\head{2}{Usage}
The \cflink{define-class} command defines a \s{class-name} as an abbreviation
for some \slink{../misc/names}{names}.  Each \s{class}, other than ones
beginning with \fq{copy} or \fq{eval}, also serves as an abbreviation for the
names of facts matching a certain criterion.  For example,
\f{contains-operator(+)} describes the collection of all facts containing the
operator \f{+}.
\p
A \s{class-name} (or \s{class}) appearing in an argument to a command other
than \clink{define-class} is replaced by its definition (or evaluated) when the
command is executed.  Evaluation in the \cflink{define-class} command can be
forced using one of the following operators:
\begin{description}
\dt \f{copy($class)}
\dd 
Replaces the \s{class-name} \f{$class} by its current definition
\p
\dt \f{eval(exp)}              
\dd 
Replaces the \s{names} \fq{exp} by an explicit list of the names of all facts
in LP's logical system that match \fq{exp}
\end{description}

\head{2}{See also}
\begin{itemize}
\item Assigning a \slink{../misc/name}{name} to a fact
\item Using \slink{../misc/names}{names} in commands
\end{itemize}