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>*debugger-hook* | Common Lisp Nova Spec</TITLE
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>Common Lisp Nova Spec</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="9_Conditions.html"
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>9. Conditions</A
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="v_debugger-hook.html"
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>*debugger-hook*</A
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>Apropos </SPAN
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><INPUT ID="apropos" AUTOFOCUS="AUTOFOCUS" PLACEHOLDER="Type here to search" ONINPUT="aproposInput(this);" ONKEYUP="aproposKeyup(event);" ONCHANGE="aproposChange(this);" ONFOCUS="aproposFocus(this);" ONFOCUSOUT="aproposFocusout(this);"
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><DIV CLASS="matter"
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><DIV CLASS="com"
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><DIV CLASS="begincom"
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><HR
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TR
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="*debugger-hook*"
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></SPAN
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><SPAN CLASS="cmsy"
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>*</SPAN
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><B
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>debugger-hook</B
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><SPAN CLASS="cmsy"
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>*</SPAN
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Variable</I
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></TD
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></TR
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></TABLE
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><HR
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></DIV
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Value Type</B
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></DT
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><DD
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><P CLASS="j"
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>a <A HREF="26_1_Glossary.html#designator"
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><EM CLASS="term"
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>designator</EM
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></A
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> for a <A HREF="26_1_Glossary.html#function"
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><EM CLASS="term"
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>function</EM
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></A
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> of two <A HREF="26_1_Glossary.html#argument"
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><EM CLASS="term"
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>arguments</EM
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></A
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> (a <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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> and the <A HREF="26_1_Glossary.html#value"
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><EM CLASS="term"
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>value</EM
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></A
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> of <A HREF="v_debugger-hook.html" CLASS="misc"
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><B
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>*debugger-hook*</B
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></A
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> at the time the debugger was entered), or <SPAN CLASS="misc"
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><B
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>nil</B
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></SPAN
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>.</P
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></DD
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><DT
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><B
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>Initial Value</B
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></DT
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><DD
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><P CLASS="j"
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><SPAN CLASS="misc"
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><B
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>nil</B
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></SPAN
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>.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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>When the <A HREF="26_1_Glossary.html#value"
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><EM CLASS="term"
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>value</EM
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></A
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> of <A HREF="v_debugger-hook.html" CLASS="misc"
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><B
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>*debugger-hook*</B
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></A
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> is <A HREF="26_1_Glossary.html#non-nil"
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><EM CLASS="term"
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>non-nil</EM
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></A
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>, it is called prior to normal entry into the debugger, either due to a call to <A HREF="f_invoke-debugger.html" CLASS="funref"
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><B
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>invoke-debugger</B
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></A
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> or due to automatic entry into the debugger from a call to <A HREF="f_error.html" CLASS="funref"
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><B
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>error</B
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></A
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> or <A HREF="f_cerror.html" CLASS="funref"
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><B
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>cerror</B
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></A
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> with a condition that is not handled. The <A HREF="26_1_Glossary.html#function"
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><EM CLASS="term"
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>function</EM
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></A
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> may either handle the <A HREF="26_1_Glossary.html#condition"
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><EM CLASS="term"
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>condition</EM
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></A
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> (transfer control) or return normally (allowing the standard debugger to run). To minimize recursive errors while debugging, <A HREF="v_debugger-hook.html" CLASS="varref"
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><B
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>*debugger-hook*</B
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></A
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> is bound to <SPAN CLASS="misc"
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><B
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>nil</B
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></SPAN
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> by <A HREF="f_invoke-debugger.html" CLASS="funref"
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><B
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>invoke-debugger</B
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></A
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> prior to calling the <A HREF="26_1_Glossary.html#function"
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><EM CLASS="term"
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>function</EM
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></A
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>.</P
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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> (defun one-of (choices &optional (prompt "Choice"))
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(let ((n (length choices)) (i))
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(do ((c choices (cdr c)) (i 1 (+ i 1)))
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((null c))
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(format t "~&[~D] ~A~%" i (car c)))
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(do () ((typep i `(integer 1 ,n)))
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(format t "~&~A: " prompt)
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(setq i (read))
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(fresh-line))
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(nth (- i 1) choices)))
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(defun my-debugger (condition me-or-my-encapsulation)
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(format t "~&Fooey: ~A" condition)
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(let ((restart (one-of (compute-restarts))))
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(if (not restart) (error "My debugger got an error."))
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(let ((*debugger-hook* me-or-my-encapsulation))
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(invoke-restart-interactively restart))))
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(let ((*debugger-hook* #'my-debugger))
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(+ 3 'a))
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Fooey: The argument to +, A, is not a number.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> [1] Supply a replacement for A.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> [2] Return to Cloe Toplevel.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Choice: 1
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Form to evaluate and use: (+ 5 'b)
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Fooey: The argument to +, B, is not a number.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> [1] Supply a replacement for B.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> [2] Supply a replacement for A.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> [3] Return to Cloe Toplevel.
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Choice: 1
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<SPAN CLASS="cmr"
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>⊳</SPAN
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> Form to evaluate and use: 1
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<SPAN CLASS="cmsy"
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><SPAN CLASS="arrow"
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>→</SPAN
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></SPAN
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> 9</PRE
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></DD
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><DT
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><B
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>Affected By</B
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></DT
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><DD
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><P CLASS="j"
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><A HREF="f_invoke-debugger.html" CLASS="funref"
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><B
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>invoke-debugger</B
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></A
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></P
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></DD
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><DT
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><B
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>Notes</B
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></DT
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><DD
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><P CLASS="j"
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>When evaluating code typed in by the user interactively, it is sometimes useful to have the hook function bind <A HREF="v_debugger-hook.html" CLASS="varref"
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><B
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>*debugger-hook*</B
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></A
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> to the <A HREF="26_1_Glossary.html#function"
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><EM CLASS="term"
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>function</EM
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></A
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> that was its second argument so that recursive errors can be handled using the same interactive facility.</P
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></DD
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></DL
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></DIV
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></DIV
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><DIV CLASS="footer"
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><DIV CLASS="btmnav"
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><A HREF="f_break.html" CLASS="prev"
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>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
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