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<h3 class="section">2.2 Concrete Syntax</h3>

<p>The previous section has developed a basic vocabulary for statements
such as &ldquo;the virtual machine code for the identity function is
<a name="index-identity-function-146"></a><code>(nil,(nil,nil))</code>&rdquo;, which are elaborated extensively in the
subsequent sections on code and data formats. However, a description in
this style would be inadequate without an explanation of how such an
entity as <code>(nil,(nil,nil))</code> is communicated to <code>avram</code> in a
virtual machine code file. The purpose of this section is to fill the
gap by explaining exactly how any given tree would be transformed to its
concrete representation.

   <p>The syntax is based on a conversion of the trees to bit strings,
<a name="index-bit-strings-147"></a>followed by grouping the bits into blocks of six, which are then encoded
by printable characters. Although anyone is free to modify <code>avram</code>,
it is recommended that the concrete syntax described here be maintained
for the sake of portability of virtual machine code applications.

   <p>Building a tree by reading the data from a file requires a more
difficult algorithm than the one presented in this section, and is not
considered because it's not strictly necessary for a
compiler. Procedures for both reading and writing are available to C and
C++ users as part of the <code>avram</code> library, and are also easily
invoked on the virtual code level.

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<li><a accesskey="1" href="Bit-String-Encoding.html#Bit-String-Encoding">Bit String Encoding</a>
<li><a accesskey="2" href="Blocking.html#Blocking">Blocking</a>
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