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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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<TITLE>19.6. By-Position Constructor Functions</TITLE>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3702 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3700 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3694 HREF="node173.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3704 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3705 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3703 HREF="node175.html"> Structures of Explicitly </A>
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<B>Up:</B> <A NAME=tex2html3701 HREF="node168.html"> Structures</A>
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<B> Previous:</B> <A NAME=tex2html3695 HREF="node173.html"> Defstruct Options</A>
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<HR> <P>
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<H1><A NAME=SECTION002360000000000000000>19.6. By-Position Constructor Functions</A></H1>
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<P>
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<A NAME=DEFSTRUCTCONSTRUCTORSYNTAX>If</A>
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the <tt>:constructor</tt> option is given as
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<tt>(<tt>:constructor</tt> <i>name</i> <i>arglist</i>)</tt>,
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then instead of making a keyword-driven constructor function,
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<tt>defstruct</tt> defines a ``positional'' constructor function,
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taking arguments whose meaning is determined by the argument's position
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rather than by a keyword.
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The <i>arglist</i> is used to describe what the arguments to the
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constructor will be. In the simplest case something like
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<tt>(<tt>:constructor</tt> make-foo (a b c))</tt> defines <tt>make-foo</tt> to be
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a three-argument constructor function whose arguments are used to initialize the
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slots named <tt>a</tt>, <tt>b</tt>, and <tt>c</tt>.
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<P>
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In addition, the keywords <tt>&optional</tt>, <tt>&rest</tt>, and <tt>&aux</tt> are
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recognized in the argument list. They work in the way you might expect,
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but there are a few fine points worthy of explanation.
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Consider this example:
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<P><pre>
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(<tt>:constructor</tt> create-foo
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(a &optional b (c 'sea) &rest d &aux e (f 'eff)))
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</pre><P>
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This defines <tt>create-foo</tt> to be a constructor of one or more arguments.
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The first argument is used to initialize the <tt>a</tt> slot. The second
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argument is used to initialize the <tt>b</tt> slot. If there isn't any
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second argument, then the default value given in the body of the
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<tt>defstruct</tt> (if given) is used instead. The third argument is used to
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initialize the <tt>c</tt> slot. If there isn't any third argument, then the
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symbol <tt>sea</tt> is used instead. Any arguments following the third
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argument are collected into a list and used to initialize the <tt>d</tt>
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slot. If there are three or fewer arguments, then <tt>nil</tt> is placed in
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the <tt>d</tt> slot. The <tt>e</tt> slot <i>is not initialized</i>; its initial
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value is undefined. Finally, the <tt>f</tt> slot is initialized to contain
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the symbol <tt>eff</tt>.
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<P>
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The actions taken in the <tt>b</tt> and <tt>e</tt> cases were carefully
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chosen to allow the user to specify all possible behaviors. Note that
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the <tt>&aux</tt> ``variables'' can be used to completely override the default
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initializations given in the body.
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<P>
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With this definition, one can write
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<P><pre>
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(create-foo 1 2)
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</pre><P>
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instead of
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<P><pre>
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(make-foo <tt>:a</tt> 1 <tt>:b</tt> 2)
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</pre><P>
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and of course <tt>create-foo</tt> provides defaulting different
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from that of <tt>make-foo</tt>.
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<P>
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It is permissible to use the
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<tt>:constructor</tt> option more than once, so that you can define several
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different constructor functions, each taking different parameters.
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<P>
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Because a constructor of this type operates By Order of Arguments,
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it is sometimes known as a BOA constructor.
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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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(DEFSTRUCT-CONSTRUCTOR-KEY-MIXTURE) <A NAME=18920> </A>
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to allow <tt>&key</tt> and <tt>&allow-other-keys</tt>
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in the
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parameter list of a ``positional'' constructor. The initialization of slots
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corresponding to keyword parameters
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is performed in the same manner as for <tt>&optional</tt> parameters.
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A variant of the example shown above illustrates this:
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<P><pre>
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(<tt>:constructor</tt> create-foo
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(a &optional b (c 'sea)
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&key p (q 'cue) ((:why y)) ((:you u) 'ewe)
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&aux e (f 'eff)))
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</pre><P>
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The treatment of slots <tt>a</tt>, <tt>b</tt>, <tt>c</tt>, <tt>e</tt>, and <tt>f</tt>
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is the same as in the original example. In addition,
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if there is a <tt>:p</tt> keyword argument, it is
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used to initialize the <tt>p</tt> slot; if there isn't any
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<tt>:p</tt> keyword argument, then the default value given in the body of the
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<tt>defstruct</tt> (if given) is used instead. Similarly,
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if there is a <tt>:q</tt> keyword argument, it is
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used to initialize the <tt>q</tt> slot; if there isn't any
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<tt>:q</tt> keyword argument, then
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the symbol <tt>cue</tt> is used instead.
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<P>
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In order thoroughly to flog this presumably already dead horse,
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we further observe that if there is a <tt>:why</tt> keyword argument, it is
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used to initialize the <tt>y</tt> slot; otherwise
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the default value for slot <tt>y</tt> is used instead. Similarly,
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if there is a <tt>:you</tt> keyword argument, it is
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used to initialize the <tt>u</tt> slot; otherwise
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the symbol <tt>ewe</tt> is used instead.
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<P>
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If memory serves me correctly, <tt>defstruct</tt> was included in the original
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design for Common Lisp some time before keyword arguments were approved.
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The failure of positional constructors to accept keyword arguments may well
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have been an oversight on my part; there is no logical reason to exclude
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them. I am grateful to X3J13 for rectifying this.
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<P>
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A remaining difficulty is that the possibility of keyword arguments
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renders the term ``positional constructor'' a misnomer. Worse yet,
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it ruins the term ``BOA constructor.'' I suggest that
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they continue to be called BOA constructors, as I refuse to abandon
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a good pun. (I regret appearing to have more compassion for puns than
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for horses.)
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<P>
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As part of the same vote X3J13 also changed <tt>defstruct</tt>
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to allow BOA constructors to have
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parameters (including supplied-p parameters)
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that do not correspond to any
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slot. Such parameters may be used in subsequent initialization forms in the
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parameter list. Consider this example:
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<P><pre>
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(defstruct (ice-cream-factory
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(:constructor fabricate-factory
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(&key (capacity 5)
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location
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(local-flavors
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(case location
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((hawaii) '(pineapple macadamia guava))
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((massachusetts) '(lobster baked-bean))
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((california) '(ginger lotus avocado
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bean-sprout garlic))
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((texas) '(jalapeno barbecue))))
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(flavors (subseq (append local-flavors
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'(vanilla
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chocolate
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strawberry
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pistachio
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maple-walnut
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peppermint))
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0 capacity)))))
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(capacity 3)
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(flavors '(vanilla chocolate strawberry mango)))
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</pre><P>
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<P>
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The structure type <tt>ice-cream-factory</tt> has two constructors.
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The standard constructor, <tt>make-ice-cream-factory</tt>,
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takes two keyword arguments named <tt>:capacity</tt> and <tt>:flavors</tt>.
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For this constructor, the default for the <tt>capacity</tt> slot is <tt>3</tt>
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and the default list of <tt>flavors</tt> is America's favorite threesome
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and a dark horse (not a dead one).
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The BOA constructor <tt>fabricate-factory</tt>
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accepts four different keyword arguments. The <tt>:capacity</tt>
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argument defaults to <tt>5</tt>, and the <tt>:flavors</tt> argument
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defaults in a complicated manner based on the other three.
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The <tt>:local-flavors</tt> argument may be specified directly,
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or may be allowed to default based on the <tt>:location</tt> of the factory.
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Here are examples of various factories:
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<P>
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<pre>
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(setq houston (fabricate-factory :capacity 4 :location 'texas))
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(setq cambridge (fabricate-factory :location 'massachusetts))
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(setq seattle (fabricate-factory :local-flavors '(salmon)))
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(setq wheaton (fabricate-factory :capacity 4 :location 'illinois))
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(setq pittsburgh (fabricate-factory :capacity 4))
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(setq cleveland (make-factory :capacity 4))
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(ice-cream-factory-flavors houston)
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=> (jalapeno barbecue vanilla chocolate)
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</pre><P>
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<P><pre>
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(ice-cream-factory-flavors cambridge)
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=> (lobster baked-bean vanilla chocolate strawberry)
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(ice-cream-factory-flavors seattle)
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=> (salmon vanilla chocolate strawberry pistachio)
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(ice-cream-factory-flavors wheaton)
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=> (vanilla chocolate strawberry pistachio)
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(ice-cream-factory-flavors pittsburgh)
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=> (vanilla chocolate strawberry pistachio)
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(ice-cream-factory-flavors cleveland)
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=> (vanilla chocolate strawberry mango)
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</pre><P>
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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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<BR> <HR><A NAME=tex2html3702 HREF="node175.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3700 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3694 HREF="node173.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3704 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3705 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3703 HREF="node175.html"> Structures of Explicitly </A>
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<B>Up:</B> <A NAME=tex2html3701 HREF="node168.html"> Structures</A>
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<B> Previous:</B> <A NAME=tex2html3695 HREF="node173.html"> Defstruct Options</A>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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