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<TITLE>CLHS: Issue SLOT-VALUE-METACLASSES Writeup</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss319_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss320.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss321_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue SLOT-VALUE-METACLASSES Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss320.htm">SLOT-VALUE-METACLASSES</A><P>
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<B>References:</B> <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A> (CLOS chapter 2)<P>
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<A REL=DEFINITION HREF="../Body/f_slt_un.htm#slot-unbound"><B>SLOT-UNBOUND</B></A><P>
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<A REL=DEFINITION HREF="../Body/f_slt_ma.htm#slot-makunbound"><B>SLOT-MAKUNBOUND</B></A><P>
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<A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A><P>
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<A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A><P>
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Related issues: Issue <A HREF="iss049.htm">CLOS-CONDITIONS</A><P>
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Issue <A HREF="iss048.htm">CLOS-CONDITIONS-AGAIN</A><P>
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<B>Category:</B> CLARIFICATION, CHANGE<P>
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<B>Edit history:</B> V1, 09 May 90, Sandra Loosemore<P>
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V2, 13 May 90, Sandra Loosemore (update discussion)<P>
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V3, 29 May 90, Sandra Loosemore (clean up)<P>
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V4, 1 Jun 90, Gabriel (only proposal SPECIFY)<P>
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V5, 06 Jun 90, Sandra Loosemore (three proposals)<P>
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V6, 11 Mar 91, Sandra Loosemore (one proposal)<P>
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V7, 26 Mar 91, Sandra Loosemore (amendment from meeting)<P>
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<B>Status:</B> V7 (proposal LESS-MINIMAL) accepted by X3J13, Mar 1991<P>
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<P>
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<P>
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<B>Problem description:<P>
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</B><P>
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The description of the function <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A> in CLOS chapter 2<P>
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requires it to be implemented using SLOT-VALUE-USING-CLASS, which<P>
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is not defined in the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A>. Because of this omission, it is<P>
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not clear on what metaclasses <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A> is defined.<P>
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<P>
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There are similar problems with the description of <A REL=DEFINITION HREF="../Body/f_slt_un.htm#slot-unbound"><B>SLOT-UNBOUND</B></A><P>
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(references SLOT-VALUE-USING-CLASS), <A REL=DEFINITION HREF="../Body/f_slt_ma.htm#slot-makunbound"><B>SLOT-MAKUNBOUND</B></A> (references<P>
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SLOT-MAKUNBOUND-USING-CLASS), <A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A> (references <P>
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SLOT-BOUNDP-USING-CLASS) and <A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A> (references <P>
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SLOT-EXISTS-P-USING-CLASS, which is not even in the current MOP<P>
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draft).<P>
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<P>
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It is stated in CLOS chapter 2 (in the section "Integrating Types <P>
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and Classes") that calling <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A> on an instance of a built-in <P>
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<A REL=DEFINITION HREF="../Body/t_class.htm#class"><B>class</B></A> signals an error, but nothing is said here about the other <P>
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functions.<P>
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<P>
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<P>
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<B>Proposal (SLOT-VALUE-METACLASSES:LESS-MINIMAL):<P>
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</B><P>
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(1) Remove references to SLOT-VALUE-USING-CLASS, <P>
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SLOT-MAKUNBOUND-USING-CLASS, SLOT-BOUNDP-USING-CLASS, and <P>
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SLOT-EXISTS-P-USING-CLASS from the language specification. Add<P>
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implementation notes to the description of <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A>, <A REL=DEFINITION HREF="../Body/f_slt_un.htm#slot-unbound"><B>SLOT-UNBOUND</B></A>,<P>
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<A REL=DEFINITION HREF="../Body/f_slt_ma.htm#slot-makunbound"><B>SLOT-MAKUNBOUND</B></A>, <A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A>, and <A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A> which reference<P>
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the corresponding functions in the metaobject protocol document.<P>
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<P>
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(2) Clarify that the function <A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A> may be called with any<P>
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object as its first argument, returning true if the object has a <P>
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slot of the given name and false otherwise.<P>
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<P>
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(3) Clarify that the <A REL=DEFINITION HREF="../Body/07_ffb.htm#standard"><B>standard</B></A> defines the behavior of <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A>, <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> of<P>
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<A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A>, <A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A>, and <A REL=DEFINITION HREF="../Body/f_slt_ma.htm#slot-makunbound"><B>SLOT-MAKUNBOUND</B></A> according to<P>
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the metaclass of their first argument, as follows:<P>
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<A REL=DEFINITION HREF="../Body/t_std_cl.htm#standard-class"><B>STANDARD-CLASS</B></A> -- all four functions work as already specified.<P>
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<A REL=DEFINITION HREF="../Body/t_built_.htm#built-in-class"><B>BUILT-IN-CLASS</B></A> -- all four functions signal an error.<P>
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<P>
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(4) Specify that the consequences are unspecified if any of the functions<P>
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<A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A>, <A REL=DEFINITION HREF="../Body/a_setf.htm#setf"><B>SETF</B></A> of <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A>, <A REL=DEFINITION HREF="../Body/f_slt_bo.htm#slot-boundp"><B>SLOT-BOUNDP</B></A>, or <A REL=DEFINITION HREF="../Body/f_slt_ma.htm#slot-makunbound"><B>SLOT-MAKUNBOUND</B></A> are<P>
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called on objects of any metaclass other than <A REL=DEFINITION HREF="../Body/t_std_cl.htm#standard-class"><B>STANDARD-CLASS</B></A> or<P>
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<A REL=DEFINITION HREF="../Body/t_built_.htm#built-in-class"><B>BUILT-IN-CLASS</B></A>. An error might be signaled in this situation. Note<P>
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in particular that the behavior of these functions on condition<P>
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objects and objects of metaclass <A REL=DEFINITION HREF="../Body/t_stu_cl.htm#structure-class"><B>STRUCTURE-CLASS</B></A> is unspecified.<P>
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<P>
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Rationale for proposal LESS-MINIMAL:<P>
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<P>
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Point by point:<P>
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<P>
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(1) This gets rid of dangling pointers into hyperspace.<P>
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<P>
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(2) This corresponds to the current MOP draft. It is also just what<P>
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88-002R would say if the remark mentioning SLOT-EXISTS-P-USING-CLASS<P>
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were removed.<P>
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<P>
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(3) This is clearly the "right" behavior.<P>
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<P>
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(4) This constrains <A REL=DEFINITION HREF="../Body/f_slt_va.htm#slot-value"><B>SLOT-VALUE</B></A> and friends from causing "crash and burn"<P>
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errors, but doesn't overconstrain implementors.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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I don't know.<P>
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<P>
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<B>Cost to Implementors:<P>
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</B><P>
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Probably small for any of the proposals.<P>
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<P>
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<B>Cost to Users:<P>
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</B><P>
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Probably none.<P>
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<P>
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<B>Cost of non-adoption:<P>
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</B><P>
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Part of the language specification will be left vague and<P>
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confusing.<P>
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<P>
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<B>Performance impact:<P>
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</B><P>
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None.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The language specification is made more concise.<P>
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<P>
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<B>Esthetics:<P>
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</B><P>
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Making this point clear would be an improvement over leaving<P>
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it unspecified.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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This writeup has undergone a lot of revision because both the MOP <P>
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document and people's perceptions of it have also been undergoing change.<P>
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It has been difficult to find a balance between not permitting "crash <P>
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and burn" errors and giving implementations freedom to support <P>
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additional behaviors via the MOP.<P>
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<P>
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Gregor says:<P>
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<P>
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The MOP draft says very little about <A REL=DEFINITION HREF="../Body/t_stu_cl.htm#structure-class"><B>STRUCTURE-CLASS</B></A>. What it used to<P>
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say about <A REL=DEFINITION HREF="../Body/t_stu_cl.htm#structure-class"><B>STRUCTURE-CLASS</B></A> was removed in response to concern that it<P>
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would make it more difficult to build delivery systems (a concern which<P>
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I don't share by the way). But, one thing the current draft does say is<P>
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that <A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A> won't ever signal an error.<P>
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<P>
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I guess my suggestion for X3J13 would be to be compatible with the MOP<P>
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draft on <A REL=DEFINITION HREF="../Body/f_slt_ex.htm#slot-exists-p"><B>SLOT-EXISTS-P</B></A> and "allow implementations to extend" what<P>
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happens when the slot-xxx functions are called on <A REL=DEFINITION HREF="../Body/f_docume.htm#structure"><B>structure</B></A> and built-in<P>
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instances.<P>
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<P>
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It's assumed that, for item (1), the editor will make the implementation<P>
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notes say something that agrees with the current MOP document.<P>
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</PRE>
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