ow vector can be used in a matrix expression.
For example, the following code is valid:</p>
<pre class="stan"><code>vector[2] vX = [ 1, 10 ]&#39;;
row_vector[2] vY = [ 100, 1000 ];
matrix[3,2] m2 = [ vX&#39;, vY, [ 1, 2 ]  ];</code></pre>
<div id="no-empty-vector-or-matrix-expressions" class="section level4 unnumbered">
<h4>No empty vector or matrix expressions</h4>
<p>The empty expression <code>[ ]</code> is ambiguous and therefore is not
allowed and similarly expressions such as <code>[ [ ] ]</code> or
<c