Document the module-provider extension to REQUIRE.

A more extensive description of modular arithmetic can be found in the

paper ``Efficient Hardware Arithmetic in Common Lisp'' by Alexey

Dejneka, and Christophe Rhodes, to be published.

\section{Extension to REQUIRE}

\cindex{require}

The behavior of \code{require} when called with only one argument is

implementation-defined.  In \cmucl, functions from the list

\var{*module-provider-functions*} are called in order with the

stringified module name as the argument.  The first function to return

non-\var{NIL} is assumed to have loaded the module.

By default the functions \code{module-provide-cmucl-defmodule} and

\code{module-provide- cmucl-library} are on this list of functions, in

that order.

\begin{defvar}{ext:}{module-provider-functions}

  This is a list of functions taking a single argument.

  \code{require} calls each function in turn with the stringified

  module name.  The first function to return non-\var{NIL} indicates

  that the module has been loaded.  The remaining functions, if any,

  are not called.

  To add new providers, push the new provider function onto the

  beginning of this list.

\end{defvar}

\begin{defmac}{ext:}{defmodule}{name \amprest{} files}

  Defines a module by registering the files that need to be loaded

  when the module is required.  If \var{name} is a symbol, its print

  name is used after downcasing it.

\end{defmac}

\begin{defun}{ext:}{module-provide-cmucl-defmodule}{module-name}

  This function is the module-provider for modules registered by a

  \code{ext:defmodule} form.  

\end{defun}

\begin{defun}{ext:}{module-provide-cmucl-library}{module-name}

  This function is the module-provider for \cmucl's libraries,

  including Gray streams, simple streams, CLX, CLM, Hemlock,

  \emph{etc}.

  This function causes a file to be loaded whose name is formed by

  merging the search-list ``modules:'' and the concatenation of

  module-name with the suffix ``-LIBRARY''.  Note that both the

  module-name and the suffix are each, separately, converted from

  :case :common to :case :local.  This merged name will be probed with

  both a .lisp and .fasl extensions, calling \code{LOAD} if it exists.

\end{defun}