an index is supplied to
an array, it returns the value at that index. When more than one
index is supplied, this indexing operation is chained. For example, if
<code>a</code> is a two-dimensional array, then <code>a[m, n]</code> is just
a convenient shorthand for <code>a[m][n]</code>.</p>
<p>Vectors, matrices, and arrays are not assignable to one another, even
if their dimensions are identical.</p>
<p>For constructing arrays in Stan, see <a href="vector-matrix-array-expressions.html#vector-matrix-array-expr