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177 lines
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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>28.1.6.2. Introduction to Methods</TITLE>
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<meta name="description" value=" Introduction to Methods">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5147 HREF="node280.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5145 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5139 HREF="node278.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5149 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5150 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5148 HREF="node280.html"> Agreement on Parameter </A>
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<B>Up:</B> <A NAME=tex2html5146 HREF="node277.html"> Generic Functions and </A>
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<B> Previous:</B> <A NAME=tex2html5140 HREF="node278.html"> Introduction to Generic </A>
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<HR> <P>
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<H3><A NAME=SECTION003216200000000000000>28.1.6.2. Introduction to Methods</A></H3>
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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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<A NAME=IntroductiontoMethodsSECTION>A</A>
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method object contains a method function, a sequence of <i>parameter specializers</i> that specify when the given method is
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applicable, a lambda-list, and a sequence of <i>qualifiers</i> that
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are used by the method combination facility to distinguish among
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methods.
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<P>
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A method object is not a function and cannot be invoked as a function.
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Various mechanisms in the Object System take a method object and invoke its method
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function, as is the case when a generic function is invoked. When this
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occurs it is said that the method is invoked or called.
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<P>
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A method-defining form contains the code that is to be run when the
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arguments to the generic function cause the method that it defines to
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be invoked. When a method-defining form is evaluated, a method object
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is created and one of four actions is taken:
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<P>
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<UL><LI> If a generic function of the given name already exists
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and if a method object already exists that agrees with the new one on
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parameter specializers and qualifiers, the new method object replaces
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the old one. For a definition of one method agreeing with another on
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parameter specializers and qualifiers, see
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section <A HREF="node280.html#AgreementonParameterSpecializersandQualifiersSECTION">28.1.6.3</A>.
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<P>
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<LI> If a generic function of the given name already exists
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and if there is no method object that agrees with the new one on
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parameter specializers and qualifiers, the existing generic function
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object is modified to contain the new method object.
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<P>
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<LI> If the given name names a non-generic function, a macro,
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or a special form, an error is signaled.
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<P>
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<LI> Otherwise a generic function is created with the methods
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specified by the method-defining form.
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<P>
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</UL>
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<P>
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If the lambda-list of a new method is not congruent with the lambda-list
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of the generic function, an error is signaled. If a
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method-defining form that cannot specify generic function options
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creates a new generic function, a lambda-list for that generic
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function is derived from the lambda-lists of the methods in the
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method-defining form in such a way as to be congruent with them. For
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a discussion of <i>congruence</i>, see
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section <A HREF="node281.html#CongruentLambdaListsforAllMethodsofaGenericFunctionSECTION">28.1.6.4</A>.
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<P>
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Each method has a <i>specialized lambda-list</i>, which determines
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when that method can be applied. A specialized lambda-list is like
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an ordinary lambda-list except that a <i>specialized parameter</i>
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may occur instead of the name of a required parameter. A specialized parameter
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is a list <tt>(<i>variable-name parameter-specializer-name</i>)</tt>,
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where <i>parameter-specializer-name</i> is either
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a name that names a class or a list <tt>(<tt>eql</tt> <i>form</i>)</tt>.
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A parameter specializer name denotes a parameter specializer as follows:
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<P>
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<UL><LI> A name that names a class denotes that class.
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<P>
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<LI> The list <tt>(<tt>eql</tt> <i>form</i>)</tt> denotes the type specifier
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<tt>(<tt>eql</tt> <i>object</i>)</tt>, where <i>object</i> is the result of
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evaluating <i>form</i>. The form <i>form</i> is evaluated in the
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lexical environment in which the method-defining form is
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evaluated. Note that <i>form</i> is evaluated only once, at the time
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the method is defined, not each time the generic function is called.
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</UL>
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<P>
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Parameter specializer names are used in macros intended as the
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user-level interface (<tt>defmethod</tt>), while parameter specializers
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are used in the functional interface.
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<P>
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[It is very important to understand clearly the distinction made
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in the preceding paragraph. A parameter specializer name
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has the form of a type specifier but is semantically quite different
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from a type specifier: a parameter specializer name of the form
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<tt>(<tt>eql</tt> <i>form</i>)</tt> is not a type specifier, for it contains
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a <i>form</i> to be evaluated. Type specifiers
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never contain forms to be evaluated. All parameter specializers
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(as opposed to parameter specializer names) are valid type specifiers,
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but not all type specifiers are valid parameter specializers. Macros such as <tt>defmethod</tt>
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take parameter specializer names and treat them as specifications for
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constructing certain type specifiers (parameter specializers) that may then be used
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with such functions as <tt>find-method</tt>.-GLS]
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<P>
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Only required parameters may be specialized, and there must be a
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parameter specializer for each required parameter. For notational
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simplicity, if some required parameter in a specialized lambda-list in
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a method-defining form is simply a variable name, its parameter
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specializer defaults to the class named <tt>t</tt>.
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<P>
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Given a generic function and a set of arguments, an <i>applicable
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method</i> is a method for that generic function whose parameter
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specializers are satisfied by their corresponding arguments. The
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following definition specifies what it means for a method to be
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applicable and for an argument to satisfy a parameter specializer.
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<P>
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Let <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43517.gif"> be the required
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arguments to a generic function in order. Let <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43519.gif"> be the parameter specializers corresponding to
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the required parameters of the method <i>M</i> in order. The method <i>M</i> is
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<i>applicable</i> when each <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43543.gif">
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<i>satisfies</i> <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif">.
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If <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif"> is a class,
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and if <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43543.gif"> is an instance of a class
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<i>C</i>, then it is said that <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43543.gif"> <i>satisfies</i>
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif"> when <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43533.gif"> or when <i>C</i> is a subclass of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif">. If
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif"> is of the form
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<tt>(<tt>eql</tt> <i>object</i>)</tt>, then it is said that
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43543.gif"> satisfies <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43523.gif">
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when the function <tt>eql</tt> applied to
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43543.gif"> and <i>object</i> is true.
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<P>
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Because a parameter specializer is a type specifier, the function
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<tt>typep</tt> can be used during method selection to determine whether an
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argument satisfies a parameter specializer. In general a
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parameter specializer cannot be a type specifier list, such as
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<tt>(<tt>vector single-float</tt>)</tt>. The only parameter specializer that can
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be a list is <tt>(<tt>eql</tt> <i>object</i>)</tt>. This requires that
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Common Lisp define the type specifier <tt>eql</tt>
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as if the following were evaluated:
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<P>
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<P><pre>
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(deftype eql (<i>object</i>) `(member ,<i>object</i>))
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</pre><P>
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[See section <A HREF="node47.html#PREDICATINGTYPESPECIFIERSSECTION">4.3</A>.-GLS]
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<P>
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A method all of whose parameter specializers are the class named
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<tt>t</tt> is called a <i>default method</i>; it is always applicable but
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may be shadowed by a more specific method.
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<P>
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Methods can have <i>qualifiers</i>, which give the method combination
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procedure a way to distinguish among methods. A method that has one
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or more qualifiers is called a <i>qualified</i> method.
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A method with no qualifiers is called an <i>unqualified method</i>.
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A qualifier is any object other than a list, that is,
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any non-<tt>nil</tt> atom. The qualifiers defined by standard method combination
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and by the built-in method combination types are symbols.
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<P>
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In this specification, the terms <i>primary method</i> and <i>auxiliary method</i> are used to partition methods within a method
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combination type according to their intended use. In standard method
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combination, primary methods are unqualified methods, and auxiliary
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methods are methods with a single qualifier that is one of
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<tt>:around</tt>, <tt>:before</tt>, or <tt>:after</tt>. When a method combination
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type is defined using the short form of
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<tt>define-method-combination</tt>, primary methods are methods qualified with
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the name of the type of method combination, and auxiliary methods have
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the qualifier <tt>:around</tt>. Thus the terms <i>primary method</i>
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and <i>auxiliary method</i> have only a relative definition within a
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given method combination type.
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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=tex2html5147 HREF="node280.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5145 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5139 HREF="node278.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5149 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5150 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5148 HREF="node280.html"> Agreement on Parameter </A>
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<B>Up:</B> <A NAME=tex2html5146 HREF="node277.html"> Generic Functions and </A>
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<B> Previous:</B> <A NAME=tex2html5140 HREF="node278.html"> Introduction to Generic </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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