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<h3 class="section">3.1 Lists</h3>

<p>The basic data structure used for representing virtual code and data in
the <code>avram</code> library is declared as a <code>list</code>. 
<a name="index-lists-396"></a><a name="index-g_t_0040code_007bhead_007d-field-397"></a><a name="index-g_t_0040code_007btail_007d-field-398"></a>The <code>list</code> type is a pointer to a structure having a <code>head</code>
field and a <code>tail</code> field, which are also lists. The empty tree,
<code>nil</code>, is represented by the C constant <code>NULL</code>. A tree of the
form <code>cons(</code><var>a</var><code>,</code><var>b</var><code>)</code> is represented in C as a list whose
<code>head</code> is the representation of <var>a</var> and whose
<code>tail</code> is the representation of <var>b</var>.

   <p>A number of other fields in the structure are maintained automatically
and should not be touched. For that matter, even the <code>head</code> and
<code>tail</code> fields should be considered read-only. Because of sharing,
it is almost never valid to modify a list &ldquo;in place&rdquo;, except for cases
that are already covered by library functions.

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