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\section{Export lists}
\label{sec:exportcalculus}

In the simplest case, the export list is given as a sequence of SML
symbol names such as shown in earlier examples.  But for more
complicated scenarios, CM provides a little ``set calculus'' for
writing export lists.  Formally, the export section specifies the
union of a sequence of {\em symbol sets}.  Each individual set in this
sequence can be one of the following:

\begin{itemize}\setlength{\itemsep}{0pt}
\item a singleton set given by the name of its sole member
\item the set difference of two other sets $s_1$ and $s_2$, written as $s_1
\mbox{\tt -} s_2$
\item the intersection of two sets $s_1$ and $s_2$, written as $s_1
\mbox{\tt *} s_2$
\item the union of sets $s_1, \ldots, s_n$ for $n \ge 0$, written as
$\mbox{\tt (} s_1 \ldots s_n \mbox{)}$
\item the top-level defined symbols of one of the SML source files $f$
that the given library or group consists of, written as
$\mbox{\tt source(}f\mbox{)}$
\item the top-level defined symbols of all constituent SML source
files that are not marked {\em local} (see section~\ref{sec:toolparam:sml}),
written as {\tt source(-)}
\item the exported symbols of one of the subgroups $g$, written as
$\mbox{\tt group(}g\mbox{)}$
\item the exported symbols of {\em all} subgroups, written as
{\tt group(-)}
\item the exported symbols of another library $l$ \\
In most situations, the notation for this is $\mbox{\tt library(}l\mbox{)}$.
However, notice that unlike in the case of subgroups and SML source
files, $l$ may or may not be listed as one of the members in the
current description.  For the case that $l$ is not a member, its
elaboration---determining its set of exported symbols---may require
{\em tool parameters} to be specified (see
section~\ref{sec:toolparam}).  When tool parameters are necessary,
they are given within an extra pair of parentheses following the name
$l$.  Example:
\begin{verbatim}
  library(foo/bar.cm (bind:(anchor:x value:y)))
\end{verbatim}
\noindent If $l$ is a member of the current description, then CM will
re-use the earlier elaboration result.  Tool parameters within the
export set specification are neither necessary nor permitted in this case.
\end{itemize}


\noindent Notes:
\begin{itemize}\setlength{\itemsep}{0pt}
\item Intersection has higher precedence than set difference.
\item Conditional compilation constructs (see
section~\ref{sec:preproc}) may be used within set union
costructs---both the parenthesized variety and at top level.
\end{itemize}

Using the export list calculus, it is easy to set up ``proxy''
libraries.  A proxy library $A$ is a library with a single member
which is another library $B$ so that the export lists of $A$ and $B$
coincide.  Proxy libraries are mainly used for the purpose of managing
anchor names and anchor environments (see
Section~\ref{sec:anchor:env}).  Using the $\mbox{\tt
library(}l\mbox{)}$ construct one can avoid the cumbersome repetition
of lengthy explicit export lists and, thus, improve maintainability.
