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43 lines
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<TITLE>CLHS: Section 24.1.2.1</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="24_ab.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="24_ab.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="24_abaa.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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24.1.2.1 Feature Expressions</H2> <P>
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Boolean combinations of <A REL=DEFINITION HREF="26_glo_f.htm#feature"><I>features</I></A>, called <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expressions</I></A>, are used by the <TT>#+</TT> and <TT>#-</TT> <A REL=DEFINITION HREF="26_glo_r.htm#reader_macro"><I>reader macros</I></A> in order to direct conditional <I>reading</I> of <A REL=DEFINITION HREF="26_glo_e.htm#expression"><I>expressions</I></A> by the <A REL=DEFINITION HREF="26_glo_l.htm#lisp_reader"><I>Lisp reader</I></A>. <P>
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The rules for interpreting a <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> are as follows: <P>
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<P><DL><P>
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<DT><A REL=DEFINITION HREF="26_glo_f.htm#feature"><I>feature</I></A> <P><DD>
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If a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> naming a <A REL=DEFINITION HREF="26_glo_f.htm#feature"><I>feature</I></A> is used as a <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A>, the <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> succeeds if that <A REL=DEFINITION HREF="26_glo_f.htm#feature"><I>feature</I></A> is <A REL=DEFINITION HREF="26_glo_p.htm#present"><I>present</I></A>; otherwise it fails. <P>
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<DT><TT>(not </TT><I>feature-conditional</I><TT>)</TT> <P><DD>
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A <A REL=DEFINITION HREF="a_not.htm#not"><B>not</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> succeeds if its argument <I>feature-conditional</I> fails; otherwise, it succeeds. <P>
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<DT><TT>(and </TT><I>feature-conditional</I><TT></TT><B>*</B><TT>)</TT> <P><DD>
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An <A REL=DEFINITION HREF="a_and.htm#and"><B>and</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> succeeds if all of its argument <I>feature-conditionals</I> succeed; otherwise, it fails. <P>
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<DT><TT>(or </TT><I>feature-conditional</I><TT></TT><B>*</B><TT>)</TT> <P><DD>
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An <A REL=DEFINITION HREF="a_or.htm#or"><B>or</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#feature_expression"><I>feature expression</I></A> succeeds if any of its argument <I>feature-conditionals</I> succeed; otherwise, it fails. <P>
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<P></DL><P>
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<A REL=CHILD HREF="24_abaa.htm"><H2>
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24.1.2.1.1 Examples of Feature Expressions</H2></A><P><HR>
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