ach module <tt>foo</tt>, there is 
<ul>
<li>
a header
file <tt>&lt;NTL/foo.h&gt;</tt>, found in subdirectory <tt>include</tt>,
<li>
a documentation file <tt>foo.txt</tt>,
found in subdirectory <tt>doc</tt>,
and
<li>
a source file <tt>foo.cpp</tt>, found in subdirectory <tt>src</tt>.
</ul>

<p>
Note that all of the header files for NTL modules include the header file
<tt>&lt;NTL/tools.h&gt;</tt>, and by default, this header file includes
the standard headers 
<ul>
<li>
<tt>&lt;cstdlib&gt;</tt>,
<li>
<tt>&lt;