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<TITLE>CLHS: Issue ALLOW-OTHER-KEYS-NIL Writeup</TITLE>
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<H2>Issue ALLOW-OTHER-KEYS-NIL Writeup</H2>
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<PRE><B>Issue:</B> <A HREF="iss008.htm">ALLOW-OTHER-KEYS-NIL</A><P>
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<B>Forum:</B> Cleanup<P>
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<B>References:</B> :ALLOW-OTHER-KEYS (pp62-63)<P>
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<B>Category:</B> CLARIFICATION/CHANGE<P>
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<B>Edit history:</B> 22-Aug-90, Version 1 by Pitman<P>
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23-Aug-90, Version 2 by Pitman (merge RWK's comments)<P>
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23-Aug-90, Version 3 by Pitman (merge Moon's comments)<P>
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13-Mar-91, Version 4 by Pitman (comments from JonL, Barmar)<P>
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<B>Status:</B> For Internal Discussion<P>
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<P>
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<B>Problem Description:<P>
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</B><P>
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CLtL specifies that a keyword argument pair of :ALLOW-OTHER-KEYS is<P>
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always permissible when the value is non-null, in which case all other<P>
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keyword arguments in that actual call are allowed regardless of the <P>
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formal <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> parameter). It doesn't specify any particular meaning <P>
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when the value for :ALLOW-OTHER-KEYS is null; consequently such an<P>
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actual argument pair will be in error unless there is a formal <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A><P>
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parameter of that name to receive it.<P>
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<P>
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The fact that :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> either does not work or does not<P>
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work reliably causes some coding to be more cumbersome than necessary.<P>
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For example, a function that might be written as:<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>DEFUN</B></A> FOO-MEMBER (ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> PERMISSIVE-P <A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A> OPTIONS)<P>
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(<A REL=DEFINITION HREF="../Body/f_apply.htm#apply"><B>APPLY</B></A> #'<A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> :ALLOW-OTHER-KEYS PERMISSIVE-P OPTIONS))<P>
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must in fact be written as:<P>
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(<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>DEFUN</B></A> FOO-MEMBER (ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> PERMISSIVE-P <A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A> OPTIONS)<P>
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(<A REL=DEFINITION HREF="../Body/s_if.htm#if"><B>IF</B></A> PERMISSIVE-P<P>
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(<A REL=DEFINITION HREF="../Body/f_apply.htm#apply"><B>APPLY</B></A> #'<A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> :ALLOW-OTHER-KEYS T OPTIONS)<P>
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(<A REL=DEFINITION HREF="../Body/f_apply.htm#apply"><B>APPLY</B></A> #'<A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> OPTIONS)))<P>
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because :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> is not given the `obvious' meaning.<P>
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<P>
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Some people have raised concerns about the permissibility and semantics<P>
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of :ALLOW-OTHER-KEYS (including :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>) if either<P>
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<A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> is used or an explicit <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> argument such as<P>
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ALLOW-OTHER-KEYS is specified. Some clarification about such issues<P>
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is probably warranted.<P>
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<P>
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<B>Proposal (ALLOW-OTHER-KEYS-NIL:PERMIT):<P>
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</B><P>
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1. Define that :ALLOW-OTHER-KEYS is always permitted as an argument<P>
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to any function which has specified <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> in the <A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>lambda</B></A> list of its<P>
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definition.<P>
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<P>
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If the value is non-NIL, then other non-matching keywords are<P>
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also tolerated (i.e., ignored). If the value is <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>, then other <P>
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non-matching keyword are not tolerated (i.e., an error should be<P>
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signalled in accordance with rules for argument mismatches <P>
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established by other proposals) unless <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> was used.<P>
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<P>
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2. Clarify that if the receiving argument list specifies an argument which <P>
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would be flagged by :ALLOW-OTHER-KEYS, then :ALLOW-OTHER-KEYS has both <P>
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its special-cased meaning (identifying whether additional keywords are <P>
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permitted) and its normal meaning (data flow into the function in <P>
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question).<P>
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<P>
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<B>Rationale:<P>
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</B><P>
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1. There can't be any reasonable use for :ALLOW-OTHER-KEYS as a keyword<P>
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to designate `real' information coming into the function since the <P>
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keyword is already reserved just in case the value it carries with<P>
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it will be non-NIL. As such, there is no ambiguity about the<P>
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programmer's intent in the case of :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>. Failing<P>
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to admit :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> just makes it unnecessarily clumsy<P>
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to write functions like FOO-MEMBER (see Problem Description).<P>
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It also defeats the purpose of there being a value associated with<P>
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:ALLOW-OTHER-KEYS in the first place.<P>
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<P>
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2. This allows functions which do their own keyword processing on <A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A><P>
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arguments to respect this flag, as well as being conceptually the most<P>
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uniform meaning.<P>
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<P>
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<B>Test Case:<P>
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</B><P>
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#1: ((<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>LAMBDA</B></A> (<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A>) T) :ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>)<P>
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<P>
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#2: (<A REL=DEFINITION HREF="../Body/m_defun.htm#defun"><B>DEFUN</B></A> FOO-MEMBER (ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> PERMISSIVE-P <A REL=DEFINITION HREF="../Body/03_da.htm#AMrest"><B>&REST</B></A> OPTIONS)<P>
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(<A REL=DEFINITION HREF="../Body/f_apply.htm#apply"><B>APPLY</B></A> #'<A REL=DEFINITION HREF="../Body/a_member.htm#member"><B>MEMBER</B></A> ITEM <A REL=DEFINITION HREF="../Body/a_list.htm#list"><B>LIST</B></A> :ALLOW-OTHER-KEYS PERMISSIVE-P OPTIONS))<P>
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(FOO-MEMBER 'X '(Z Y X) <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A>)<P>
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<P>
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#3: ((<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>LAMBDA</B></A> (<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> ALLOW-OTHER-KEYS) ALLOW-OTHER-KEYS)<P>
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:ALLOW-OTHER-KEYS 6 :FRED 7)<P>
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<P>
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#4: ((<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>LAMBDA</B></A> (<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> ALLOW-OTHER-KEYS) ALLOW-OTHER-KEYS)<P>
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:ALLOW-OTHER-KEYS <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> :FRED 7)<P>
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<P>
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#5: ((<A REL=DEFINITION HREF="../Body/a_lambda.htm#lambda"><B>LAMBDA</B></A> (<A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> ((:ALLOW-OTHER-KEYS FOO))) FOO)<P>
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:ALLOW-OTHER-KEYS 6 :FRED 7)<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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#1: Symbolics Genera 8.0 and Sun Common Lisp 3.0.1 signal an error.<P>
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Symbolics Genera 8.1 returns T, which is consistent with proposal <P>
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PERMIT.<P>
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<P>
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#2: Symbolics Genera 8.0 and Sun Common Lisp 3.0.1 signal an error.<P>
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Symbolics Genera 8.1 returns (X), which is consistent with proposal<P>
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PERMIT.<P>
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<P>
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#3: Sun Common Lisp 3.0.1 signals an error.<P>
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Symbolics Genera 8.0 and 8.1 return 6, which is consistent with <P>
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proposal PERMIT.<P>
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<P>
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#4: Symbolics Genera 8.0, Symbolics Genera 8.1, and Sun Common Lisp 3.0.1<P>
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signal an error, which is consistent with proposal PERMIT.<P>
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<P>
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#5: Sun Common Lisp 3.0.1 signals an error.<P>
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Symbolics Genera 8.0 and 8.1 return 6, which is consistent with<P>
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proposal PERMIT.<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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Relatively small.<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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None. Currently, a valid user program must avoid stepping on this<P>
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error situation.<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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See aesthetics.<P>
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<P>
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<B>Benefits:<P>
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</B><P>
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See aesthetics.<P>
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<P>
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<B>Aesthetics:<P>
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</B><P>
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This simplifies the user model by making the value associated with<P>
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:ALLOW-OTHER-KEYS be what controls whether other keys are permitted,<P>
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rather than by making the presence of the sequence ``:ALLOW-OTHER-KEYS T''<P>
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in the arglist what controls it. In the status quo, the description of<P>
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how :ALLOW-OTHER-KEYS works is like no other keyword in the language.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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Pitman, RWK, and Moon support this proposal.<P>
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<P>
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RWK comments: ``Together with my addenda above, [this] proposal <P>
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means that <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> without <A REL=DEFINITION HREF="../Body/03_da.htm#AMallow-other-keys"><B>&ALLOW-OTHER-KEYS</B></A> implies a "hidden" <P>
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keyword argument of :ALLOW-OTHER-KEYS that the <A REL=DEFINITION HREF="../Body/03_da.htm#AMkey"><B>&KEY</B></A> processing <P>
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checks. In all other regards, it is a perfectly ordinary keyword <P>
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argument.''<P>
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</PRE>
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