2004-01-21 07:33:47 +01:00
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>LuaSocket: Network support for the Lua language</title>
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<link rel="stylesheet" href="reference.css" type="text/css">
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</head>
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<body>
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<!-- header +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<div class=header>
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<hr>
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<center>
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<table summary="LuaSocket logo">
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<tr><td align=center><a href="http://www.lua.org">
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<img border=0 alt="LuaSocket" src="luasocket.png">
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</a></td></tr>
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<tr><td align=center valign=top>Network support for the Lua language
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</td></tr>
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</table>
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<p class=bar>
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<a href="home.html">home</a> ·
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<a href="home.html#download">download</a> ·
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<a href="introduction.html">introduction</a> ·
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<a href="reference.html">reference</a>
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</p>
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</center>
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<hr>
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</div>
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<!-- stream ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h2 id=stream>Streaming with Callbacks</h2>
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<p>
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HTTP, FTP, and SMTP transfers sometimes involve large amounts of
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information. Sometimes an application needs to generate outgoing data
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in real time, or needs to process incoming information as it is being
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received. To address these problems, LuaSocket allows HTTP and SMTP message
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bodies and FTP file contents to be received or sent through the
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callback mechanism outlined below.
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</p>
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<p>
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Instead of returning the received contents
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as a big to the Lua application, the library allows the user to
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provide a <em>receive callback</em> that will be called with successive
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chunks of data, as the data becomes available. Conversely, the <em>send
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callback</em> mechanism can be used when the application wants to incrementally provide LuaSocket with the data to be sent.
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</p>
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<!-- tohostname +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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2004-01-24 03:47:24 +01:00
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<p class=name id=receive>
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<b>receive_cb(</b>chunk, err<b>)</b>
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</p>
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<p class=description>
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A receive callback will be repeatedly called by
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LuaSocket wheneve new data is available. Each time it is called, the
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callback receives successive chunks of downloaded data.
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</p>
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<p class=parameters>
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<tt>Chunk</tt> contains the latest downloaded chunk of data.
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When the transmission is over, the function is called with an
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empty string (i.e. <tt>""</tt>) in <tt>chunk</tt>.
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If an error occurs, the function receives a <b><tt>nil</tt></b>
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<tt>chunk</tt> and an error message in <tt>err</tt>.
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</p>
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<p class=return>
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The callback can abort transmission by returning <b><tt>nil</tt></b> as its first
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return value, and an optional error message as the
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second return value. To continue receiving
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data, the function should return non-<b><tt>nil</tt></b> as its first return
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value. Optionally, in this case, it may return a
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second return value, with a new callback function to take its place.
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</p>
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<!-- send_cb ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<p class=name id=send>
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<b>send_cb()</b>
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</p>
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<p class=description>
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A send callback will be called whenever LuaSocket
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needs more data to be sent.
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</p>
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<p class=return>
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Each time the callback is called, it should return the next chunk of data. It
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can optionally return, as it's second return value, a new callback to replace
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itself. The callback can abort the process at any time by returning
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<b><tt>nil</tt></b> followed by an optional error message.
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</p>
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<!-- predefined +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<h2 id=predefined>Predefined Callbacks</h2>
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<p>
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The Callback module provides several predefined callbacks to
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perform the most common input/output operations. Callbacks are provided
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that send and receive contents of files and strings. Furthermore,
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composition functions are provided to chain callbacks with filters, such as
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the filters defined in the <a href=mime.html>MIME</a> module.
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Together, these two modules provide a powerful interface to send and
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receive information.
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</p>
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<!-- send.file ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<p class=name id=send.file>
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<b>send.file</b>(file, io_err<b>)</b>
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</p>
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<p class=description>
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This function creates a send callback that returns the contents
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of a file, chunk by chunk, until the file has been entirely sent.
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When done, the callback closes the file.
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</p>
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<p class=parameters>
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<tt>File</tt> is a file opened for reading. If <tt>file</tt> is
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<b><tt>nil</tt></b>, <tt>io_err</tt> can contain an error message. In this
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case, the function returns a callback that just aborts
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transmission with the error message.
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</p>
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<p class=return>
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Returns a send callback for the file.
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</p>
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<p class=note>
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Note: This function is designed so that it directly accepts the return
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values of the <tt>io.open</tt> function.
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</p>
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<!-- send.string ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<p class=name id=send.string>
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<b>send.string(</b>str, err<b>)</b>
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</p>
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<p class=description>
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This function creates a send callback that will send
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the contents of a string.
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</p>
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<p class=parameters>
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<tt>Str</tt> is the string to be sent.
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<!--
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If <tt>str</tt> is
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<b><tt>nil</tt></b>, <tt>err</tt> can optionally contain an error message.
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-->
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</p>
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<p class=return>
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Returns a send callback for the string, or <b><tt>nil</tt></b> if the string is
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<b><tt>nil</tt></b>.
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</p>
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<p class=note>
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Note: A <tt>nil</tt></b>
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send callback is equivalent to a callback that returns the empty string.
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</p>
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<!-- send.chain ++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<p class=name id=send.chain>
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<b>send.chain(</b>send_cb, filter<b>)</b>
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</p>
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<p class=description>
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This function creates a send callback that will filter
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all the data it receives from another send callback, before
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sending it through.
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</p>
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<p class=parameters>
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<tt>Send_cb</tt> is the send callback to be chained to <tt>filter</tt>.
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</p>
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<p class=return>
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Returns a callback chained with the filter.
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</p>
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2004-01-24 03:47:24 +01:00
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<p class=note>
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Note: Several filters are defined in the <a href=mime.html>MIME</a>
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module. Below is an example that creates a send callback that sends
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a file's contents encoded in the Base64 transfer content encoding.
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</p>
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<pre class=example>
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send_cb = socket.callback.send.chain(
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socket.callback.send.file(io.open("input.bin"))
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socket.mime.chain(
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socket.mime.encode("base64"),
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socket.mime.wrap("base64")
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)
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)
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</pre>
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<p class=note>
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The call to <a href=mime.html#chain><tt>socket.mime.chain</tt></a>
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creates a chained filter that encodes it's input and then breaks it
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into lines. The call to <tt>socket.callback.chain</tt> creates a chained
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send callback that reads the file from disk and passes it through the
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filter before sending it.
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</p>
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<!-- footer +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
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<div class=footer>
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<hr>
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<center>
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<p class=bar>
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<a href="home.html">home</a> ·
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<a href="home.html#down">download</a> ·
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<a href="introduction.html">introduction</a> ·
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<a href="reference.html">reference</a>
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</p>
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<p>
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<small>
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Last modified by Diego Nehab on <br>
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Sat Aug 9 01:00:41 PDT 2003
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</small>
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</p>
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</center>
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</div>
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</body>
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</html>
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