r5rs-z-h-7.html

scheme 标准（r5rs）。Scheme是MIT发布的基于Lambda运算的语言

&nbsp;&nbsp;&nbsp;&nbsp;(-&nbsp;3&nbsp;2)<br>
&nbsp;&nbsp;&nbsp;&nbsp;(+&nbsp;3&nbsp;2))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;1<p></tt><p>

<p><p>

<a name="%_sec_4.1.6"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.1.6">4.1.6&nbsp;&nbsp;Assignments</a></h3><p>

<p><p><p>

<p><div align=left><u>syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_102"></a>set!<i> &lt;variable&gt; &lt;expression&gt;</i>)</tt>&nbsp;</div>
<p>

&lt;Expression&gt; is evaluated, and the resulting value is stored in
the location to which &lt;variable&gt; is bound.  &lt;Variable&gt; must
be bound either in some region<a name="%_idx_104"></a> enclosing the <tt>set!</tt> expression
or at top level.  The result of the <tt>set!</tt> expression is
unspecified.<p>

<tt><p>(define&nbsp;x&nbsp;2)<br>
(+&nbsp;x&nbsp;1)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;3<br>
(set!&nbsp;x&nbsp;4)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<i>unspecified</i><br>
(+&nbsp;x&nbsp;1)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;5<p></tt><p>

<p><p>

<a name="%_sec_4.2"></a>
<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2">4.2&nbsp;&nbsp;Derived expression types</a></h2><p>

<p>

The constructs in this section are hygienic, as discussed in
section&nbsp;<a href="#%_sec_4.3">4.3</a>.
For reference purposes, section&nbsp;<a href="r5rs-Z-H-10.html#%_sec_7.3">7.3</a> gives macro definitions
that will convert most of the constructs described in this section 
into the primitive constructs described in the previous section.<p>

<p>

<a name="%_sec_4.2.1"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.1">4.2.1&nbsp;&nbsp;Conditionals</a></h3><p>

<p><p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_106"></a>cond<i> &lt;clause<sub>1</sub>&gt; &lt;clause<sub>2</sub>&gt; <tt>...</tt></i>)</tt>&nbsp;</div>
<p>

<em>Syntax: </em>Each &lt;clause&gt; should be of the form
<tt><p>(&lt;test&gt;&nbsp;&lt;expression<sub>1</sub>&gt;&nbsp;<tt>...</tt>)<p></tt>
where &lt;test&gt; is any expression.  Alternatively, a &lt;clause&gt; may be
of the form
<tt><p>(&lt;test&gt;&nbsp;=&gt;&nbsp;&lt;expression&gt;)<p></tt>
The last &lt;clause&gt; may be
an ``else clause,'' which has the form
<tt><p>(else&nbsp;&lt;expression<sub>1</sub>&gt;&nbsp;&lt;expression<sub>2</sub>&gt;&nbsp;<tt>...</tt>).<p></tt>
<a name="%_idx_108"></a>
<a name="%_idx_110"></a><p>

<em>Semantics: </em>A <tt>cond</tt> expression is evaluated by evaluating the &lt;test&gt;
expressions of successive &lt;clause&gt;s in order until one of them
evaluates to a true value<a name="%_idx_112"></a> (see
section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.3.1">6.3.1</a>).  When a &lt;test&gt; evaluates to a true
value, then the remaining &lt;expression&gt;s in its &lt;clause&gt; are
evaluated in order, and the result(s) of the last &lt;expression&gt; in the
&lt;clause&gt; is(are) returned as the result(s) of the entire <tt>cond</tt>
expression.  If the selected &lt;clause&gt; contains only the
&lt;test&gt; and no &lt;expression&gt;s, then the value of the
&lt;test&gt; is returned as the result.  If the selected &lt;clause&gt; uses the
<tt>=&gt;</tt> alternate form, then the &lt;expression&gt; is evaluated.
Its value must be a procedure that accepts one argument; this procedure is then
called on the value of the &lt;test&gt; and the value(s) returned by this
procedure is(are) returned by the <tt>cond</tt> expression.
If all &lt;test&gt;s evaluate
to false values, and there is no else clause, then the result of
the conditional expression is unspecified; if there is an else
clause, then its &lt;expression&gt;s are evaluated, and the value(s) of
the last one is(are) returned.<p>

<tt><p>(cond&nbsp;((&gt;&nbsp;3&nbsp;2)&nbsp;'greater)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((&lt;&nbsp;3&nbsp;2)&nbsp;'less))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;greater<br>
(cond&nbsp;((&gt;&nbsp;3&nbsp;3)&nbsp;'greater)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((&lt;&nbsp;3&nbsp;3)&nbsp;'less)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(else&nbsp;'equal))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;equal<br>
(cond&nbsp;((assv&nbsp;'b&nbsp;'((a&nbsp;1)&nbsp;(b&nbsp;2)))&nbsp;=&gt;&nbsp;cadr)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(else&nbsp;<tt>#f</tt>))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;2<p></tt><p>

<p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_114"></a>case<i> &lt;key&gt; &lt;clause<sub>1</sub>&gt; &lt;clause<sub>2</sub>&gt; <tt>...</tt></i>)</tt>&nbsp;</div>
<p>

<em>Syntax: </em>&lt;Key&gt; may be any expression.  Each &lt;clause&gt; should have
the form
<tt><p>((&lt;datum<sub>1</sub>&gt;&nbsp;<tt>...</tt>)&nbsp;&lt;expression<sub>1</sub>&gt;&nbsp;&lt;expression<sub>2</sub>&gt;&nbsp;<tt>...</tt>),<p></tt>
where each &lt;datum&gt; is an external representation of some object.
All the &lt;datum&gt;s must be distinct.
The last &lt;clause&gt; may be an ``else clause,'' which has the form
<tt><p>(else&nbsp;&lt;expression<sub>1</sub>&gt;&nbsp;&lt;expression<sub>2</sub>&gt;&nbsp;<tt>...</tt>).<p></tt>
<a name="%_idx_116"></a><p>

<em>Semantics: </em>A <tt>case</tt> expression is evaluated as follows.  &lt;Key&gt; is
evaluated and its result is compared against each &lt;datum&gt;.  If the
result of evaluating &lt;key&gt; is equivalent (in the sense of
<tt>eqv?</tt>; see section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.1">6.1</a>) to a &lt;datum&gt;, then the
expressions in the corresponding &lt;clause&gt; are evaluated from left
to right and the result(s) of the last expression in the &lt;clause&gt; is(are)
returned as the result(s) of the <tt>case</tt> expression.  If the result of
evaluating &lt;key&gt; is different from every &lt;datum&gt;, then if
there is an else clause its expressions are evaluated and the
result(s) of the last is(are) the result(s) of the <tt>case</tt> expression;
otherwise the result of the <tt>case</tt> expression is unspecified.<p>

<tt><p>(case&nbsp;(*&nbsp;2&nbsp;3)<br>
&nbsp;&nbsp;((2&nbsp;3&nbsp;5&nbsp;7)&nbsp;'prime)<br>
&nbsp;&nbsp;((1&nbsp;4&nbsp;6&nbsp;8&nbsp;9)&nbsp;'composite))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;composite<br>
(case&nbsp;(car&nbsp;'(c&nbsp;d))<br>
&nbsp;&nbsp;((a)&nbsp;'a)<br>
&nbsp;&nbsp;((b)&nbsp;'b))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<i>unspecified</i><br>
(case&nbsp;(car&nbsp;'(c&nbsp;d))<br>
&nbsp;&nbsp;((a&nbsp;e&nbsp;i&nbsp;o&nbsp;u)&nbsp;'vowel)<br>
&nbsp;&nbsp;((w&nbsp;y)&nbsp;'semivowel)<br>
&nbsp;&nbsp;(else&nbsp;'consonant))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;consonant<p></tt><p>

<p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_118"></a>and<i> &lt;test<sub>1</sub>&gt; <tt>...</tt></i>)</tt>&nbsp;</div>
<p>

The &lt;test&gt; expressions are evaluated from left to right, and the
value of the first expression that evaluates to a false value (see
section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.3.1">6.3.1</a>) is returned.  Any remaining expressions
are not evaluated.  If all the expressions evaluate to true values, the
value of the last expression is returned.  If there are no expressions
then <tt>#t</tt> is returned.<p>

<tt><p>(and&nbsp;(=&nbsp;2&nbsp;2)&nbsp;(&gt;&nbsp;2&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#t</tt><br>
(and&nbsp;(=&nbsp;2&nbsp;2)&nbsp;(&lt;&nbsp;2&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#f</tt><br>
(and&nbsp;1&nbsp;2&nbsp;'c&nbsp;'(f&nbsp;g))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(f&nbsp;g)<br>
(and)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#t</tt><p></tt><p>

<p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_120"></a>or<i> &lt;test<sub>1</sub>&gt; <tt>...</tt></i>)</tt>&nbsp;</div>
<p>

The &lt;test&gt; expressions are evaluated from left to right, and the value of the
first expression that evaluates to a true value (see
section&nbsp;<a href="r5rs-Z-H-9.html#%_sec_6.3.1">6.3.1</a>) is returned.  Any remaining expressions
are not evaluated.  If all expressions evaluate to false values, the
value of the last expression is returned.  If there are no
expressions then <tt>#f</tt> is returned.<p>

<tt><p>(or&nbsp;(=&nbsp;2&nbsp;2)&nbsp;(&gt;&nbsp;2&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#t</tt><br>
(or&nbsp;(=&nbsp;2&nbsp;2)&nbsp;(&lt;&nbsp;2&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#t</tt><br>
(or&nbsp;<tt>#f</tt>&nbsp;<tt>#f</tt>&nbsp;<tt>#f</tt>)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#f</tt><br>
(or&nbsp;(memq&nbsp;'b&nbsp;'(a&nbsp;b&nbsp;c))&nbsp;<br>
&nbsp;&nbsp;&nbsp;&nbsp;(/&nbsp;3&nbsp;0))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;(b&nbsp;c)<p></tt><p>

<p><p>

<a name="%_sec_4.2.2"></a>
<h3><a href="r5rs-Z-H-2.html#%_toc_%_sec_4.2.2">4.2.2&nbsp;&nbsp;Binding constructs</a></h3><p>

The three binding constructs <tt>let</tt>, <tt>let*</tt>, and <tt>letrec</tt>
give Scheme a block structure, like Algol 60.  The syntax of the three
constructs is identical, but they differ in the regions<a name="%_idx_122"></a> they establish
for their variable bindings.  In a <tt>let</tt> expression, the initial
values are computed before any of the variables become bound; in a
<tt>let*</tt> expression, the bindings and evaluations are performed
sequentially; while in a <tt>letrec</tt> expression, all the bindings are in
effect while their initial values are being computed, thus allowing
mutually recursive definitions.<p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_124"></a>let<i> &lt;bindings&gt; &lt;body&gt;</i>)</tt>&nbsp;</div>
<p>

<em>Syntax: </em>&lt;Bindings&gt; should have the form
<tt><p>((&lt;variable<sub>1</sub>&gt;&nbsp;&lt;init<sub>1</sub>&gt;)&nbsp;<tt>...</tt>),<p></tt>
where each &lt;init&gt; is an expression, and &lt;body&gt; should be a
sequence of one or more expressions.  It is
an error for a &lt;variable&gt; to appear more than once in the list of variables
being bound.<p>

<em>Semantics: </em>The &lt;init&gt;s are evaluated in the current environment (in some
unspecified order), the &lt;variable&gt;s are bound to fresh locations
holding the results, the &lt;body&gt; is evaluated in the extended
environment, and the value(s) of the last expression of &lt;body&gt;
is(are) returned.  Each binding of a &lt;variable&gt; has &lt;body&gt; as its
region.<a name="%_idx_126"></a><p>

<tt><p>(let&nbsp;((x&nbsp;2)&nbsp;(y&nbsp;3))<br>
&nbsp;&nbsp;(*&nbsp;x&nbsp;y))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;6<br>
<br>
(let&nbsp;((x&nbsp;2)&nbsp;(y&nbsp;3))<br>
&nbsp;&nbsp;(let&nbsp;((x&nbsp;7)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(z&nbsp;(+&nbsp;x&nbsp;y)))<br>
&nbsp;&nbsp;&nbsp;&nbsp;(*&nbsp;z&nbsp;x)))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;35<p></tt><p>

See also named <tt>let</tt>, section <a href="#%_sec_4.2.4">4.2.4</a>.<p>

<p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_128"></a>let*<i> &lt;bindings&gt; &lt;body&gt;</i>)</tt>&nbsp;</div>
<p>

<em>Syntax: </em>&lt;Bindings&gt; should have the form
<tt><p>((&lt;variable<sub>1</sub>&gt;&nbsp;&lt;init<sub>1</sub>&gt;)&nbsp;<tt>...</tt>),<p></tt>
and &lt;body&gt; should be a sequence of
one or more expressions.<p>

<em>Semantics: </em><tt>Let*</tt> is similar to <tt>let</tt>, but the bindings are performed
sequentially from left to right, and the region<a name="%_idx_130"></a> of a binding indicated
by <tt>(&lt;variable&gt; &lt;init&gt;)</tt> is that part of the <tt>let*</tt>
expression to the right of the binding.  Thus the second binding is done
in an environment in which the first binding is visible, and so on.<p>

<tt><p>(let&nbsp;((x&nbsp;2)&nbsp;(y&nbsp;3))<br>
&nbsp;&nbsp;(let*&nbsp;((x&nbsp;7)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(z&nbsp;(+&nbsp;x&nbsp;y)))<br>
&nbsp;&nbsp;&nbsp;&nbsp;(*&nbsp;z&nbsp;x)))&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;70<p></tt><p>

<p><p>

<p><div align=left><u>library syntax:</u>&nbsp;&nbsp;<tt>(<a name="%_idx_132"></a>letrec<i> &lt;bindings&gt; &lt;body&gt;</i>)</tt>&nbsp;</div>
<p>

<em>Syntax: </em>&lt;Bindings&gt; should have the form
<tt><p>((&lt;variable<sub>1</sub>&gt;&nbsp;&lt;init<sub>1</sub>&gt;)&nbsp;<tt>...</tt>),<p></tt>
and &lt;body&gt; should be a sequence of
one or more expressions. It is an error for a &lt;variable&gt; to appear more
than once in the list of variables being bound.<p>

<em>Semantics: </em>The &lt;variable&gt;s are bound to fresh locations holding undefined
values, the &lt;init&gt;s are evaluated in the resulting environment (in
some unspecified order), each &lt;variable&gt; is assigned to the result
of the corresponding &lt;init&gt;, the &lt;body&gt; is evaluated in the
resulting environment, and the value(s) of the last expression in
&lt;body&gt; is(are) returned.  Each binding of a &lt;variable&gt; has the
entire <tt>letrec</tt> expression as its region<a name="%_idx_134"></a>, making it possible to
define mutually recursive procedures.<p>

<tt><p>(letrec&nbsp;((even?<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(lambda&nbsp;(n)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(if&nbsp;(zero?&nbsp;n)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<tt>#t</tt><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(odd?&nbsp;(-&nbsp;n&nbsp;1)))))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(odd?<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(lambda&nbsp;(n)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(if&nbsp;(zero?&nbsp;n)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<tt>#f</tt><br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(even?&nbsp;(-&nbsp;n&nbsp;1))))))<br>
&nbsp;&nbsp;(even?&nbsp;88))&nbsp;&nbsp;&nbsp;<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;===&gt;&nbsp;&nbsp;<tt>#t</tt><p></tt><p>

One restriction on <tt>letrec</tt> is very important: it must be possible
to evaluate each &lt;init&gt; without assigning or referring to the value of any