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175 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>5.3.2. Declaring Global Variables and Named Constants</TITLE>
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<BODY>
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<meta name="description" value=" Declaring Global Variables and Named Constants">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html2346 HREF="node68.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2344 HREF="node65.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2338 HREF="node66.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2348 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2349 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2347 HREF="node68.html"> Control of Time </A>
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<B>Up:</B> <A NAME=tex2html2345 HREF="node65.html"> Top-Level Forms</A>
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<B> Previous:</B> <A NAME=tex2html2339 HREF="node66.html"> Defining Named Functions</A>
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<HR> <P>
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<H2><A NAME=SECTION00932000000000000000>5.3.2. Declaring Global Variables and Named Constants</A></H2>
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<P>
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The <tt>defvar</tt> and <tt>defparameter</tt> special forms are
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the usual means of specifying globally defined variables.
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The <tt>defconstant</tt> special form is used for defining named constants.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>defvar</tt> <tt><i>name</I></tt> <tt>[<i>initial-value</I></tt> <tt>[<i>documentation</I>]]</tt> <tt><BR>
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</tt><tt>defparameter</tt> <tt><i>name</I></tt> <tt><i>initial-value</i></tt> <tt>[<i>documentation</i>]</tt> <tt><BR>
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</tt><tt>defconstant</tt> <tt><i>name</i></tt> <tt><i>initial-value</i></tt> <tt>[<i>documentation</i>]</tt><P>
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<tt>defvar</tt> is the recommended way to declare the use
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of a special variable in a program.
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<P><pre>
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(defvar <i>variable</i>)
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</pre><P>
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proclaims <i>variable</i> to be <tt>special</tt> (see <tt>proclaim</tt>),
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and may perform other system-dependent bookkeeping actions.
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<p>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1987 (DEFVAR-INITIALIZATION) <A NAME=3592> </A> to clarify
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that if no <i>initial-value</i> form is provided, <tt>defvar</tt>
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does not change the value of the <i>variable</i>;
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if no <i>initial-value</i> form is provided and the variable
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has no value, <tt>defvar</tt> does not give it a value.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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If a second argument form is supplied,
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<P><pre>
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(defvar <i>variable</i> <i>initial-value</i>)
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</pre><P>
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then <i>variable</i> is initialized to the result of evaluating the form
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<i>initial-value</i> unless it already has a value. The <i>initial-value</i> form
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is not evaluated unless it is used; this fact is useful if
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evaluation of the <i>initial-value</i> form does something
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expensive like creating a large data structure.
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<p>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in June 1987 (DEFVAR-INIT-TIME) <A NAME=3608> </A> to clarify that
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evaluation of the <i>initial-value</i> and the initialization of the
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variable occur, if at all, at the time the <tt>defvar</tt> form is executed,
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and that the <i>initial-value</i> form is evaluated
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if and only if the <i>variable</i> does not already have a value.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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The initialization is
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performed by assignment and thus assigns a global value to the variable
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unless there are currently special bindings of that variable.
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Normally there should not be any such special bindings.
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<P>
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<tt>defvar</tt> also provides a good place to put a comment describing the
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meaning of the variable, whereas an ordinary <tt>special</tt> proclamation
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offers the
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temptation to declare several variables at once and not have room to
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describe them all.
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<P><pre>
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(defvar *visible-windows* 0
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"Number of windows at least partially visible on the screen")
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</pre><P>
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<P>
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<tt>defparameter</tt> is similar to <tt>defvar</tt>, but <tt>defparameter</tt> requires
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an <i>initial-value</i> form, always evaluates the form, and assigns the
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result to the variable. The semantic distinction is that <tt>defvar</tt>
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is intended to declare a variable changed by the program, whereas
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<tt>defparameter</tt> is intended to declare a variable that is normally
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constant but can be changed (possibly at run time), where such a change
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is considered a
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change <i>to</i> the program. <tt>defparameter</tt> therefore does not indicate
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that the quantity <i>never</i> changes; in particular, it does not license
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the compiler to build assumptions about the value into programs being
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compiled.
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<P>
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<tt>defconstant</tt> is like <tt>defparameter</tt> but <i>does</i> assert that
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the value of the variable <i>name</i> is fixed and does license
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the compiler to build assumptions about the value into programs being
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compiled. (However, if the compiler chooses to replace references
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to the name of the constant by the value of the constant in code
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to be compiled, perhaps in order to allow further optimization,
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the compiler must take care that such ``copies'' appear to be <tt>eql</tt>
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to the object that is the actual value of the constant. For example,
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the compiler may freely make copies of numbers but must exercise
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care when the value is a list.)
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<P>
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It is an error if there are any special bindings
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of the variable at the time the <tt>defconstant</tt> form
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is executed (but implementations may or may not check for this).
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<P>
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Once a name has been declared by <tt>defconstant</tt> to be constant,
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any further assignment to or binding of that special variable is an error.
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This is the case for such system-supplied constants as <tt>t</tt> and
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<tt>most-positive-fixnum</tt>.
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A compiler may also choose to issue warnings about bindings of
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the lexical variable of the same name.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(DEFCONSTANT-SPECIAL) <A NAME=3637> </A>
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to clarify the preceding paragraph by specifying
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that it is an error to rebind constant symbols
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as either lexical or special variables.
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Consequently, a valid reference to a symbol declared with <tt>defconstant</tt>
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always refers to its global value.
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(Unfortunately, this violates the principle of referential transparency,
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for one cannot always choose names for lexical variables without regard
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to surrounding context.)
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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For any of these constructs,
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the documentation should be a string. The string is attached
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to the name of the variable, parameter, or constant
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under the <tt>variable</tt> documentation type; see the <tt>documentation</tt>
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function.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in March 1988
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(DEFVAR-DOCUMENTATION) <A NAME=3643> </A>
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to clarify that the <i>documentation-string</i>
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is not evaluated but must appear as a literal string when the <tt>defvar</tt>,
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<tt>defparameter</tt>, or <tt>defconstant</tt> form is evaluated.
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<P>
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For example,
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the form
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<P><pre>
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(defvar *avoid-registers* nil "Compilation control switch #43")
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</pre><P>
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is legitimate, but
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<P><pre>
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(defvar *avoid-registers* nil
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(format nil "Compilation control switch #~D"
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(incf *compiler-switch-number*)))
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</pre><P>
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is erroneous because the call to <tt>format</tt> is not a literal string.
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<P>
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(On the other hand, the form
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<P><pre>
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(defvar *avoid-registers* nil
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#.(format nil "Compilation control switch #~D"
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(incf *compiler-switch-number*)))
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</pre><P>
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might be used to accomplish the same purpose, because the call to
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<tt>format</tt> is evaluated at <tt>read</tt> time; when the <tt>defvar</tt> form
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is evaluated, only the result of the call to <tt>format</tt>, a string,
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appears in the <tt>defvar</tt> form.)
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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These constructs are normally used only as top-level forms. The
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value returned by each of these constructs is the <i>name</i> declared.
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<P>
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<BR> <HR><A NAME=tex2html2346 HREF="node68.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2344 HREF="node65.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2338 HREF="node66.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2348 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2349 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2347 HREF="node68.html"> Control of Time </A>
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<B>Up:</B> <A NAME=tex2html2345 HREF="node65.html"> Top-Level Forms</A>
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<B> Previous:</B> <A NAME=tex2html2339 HREF="node66.html"> Defining Named Functions</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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