 <code>operator -</code> computes the difference of two
vector expressions.</p>
<h4>Definition</h4>
<p>Defined in the header vector_expression.hpp.</p>
<h4>Type requirements</h4>
<ul>
<li><code>E1</code> is a model of <a href=
"expression_concept.htm#vector_expression">Vector Expression</a> .</li>
<li><code>E2</code> is a model of <a href=
"expression_concept.htm#vector_expression">Vector Expression</a> .</li>
</ul>
<h4>Preconditions</h4>
<ul>
<li><code>e1 ().size () == e2 ().size ()</code></li>
</ul>
<h4>C