attempt to store a new value into a location that is denoted by an
immutable object.</p>
<p>
</p>
<a name="%_sec_3.5"></a>
<h2><a href="r5rs-Z-H-2.html#%_toc_%_sec_3.5">3.5&nbsp;&nbsp;Proper tail recursion</a></h2>
<p></p>
<p>
Implementations of Scheme are required to be
<em>properly tail-recursive</em><a name="%_idx_78"></a>.
Procedure calls that occur in certain syntactic
contexts defined below are `tail calls'.  A Scheme implementation is
properly tail-recursive if it supports an unbounded number of acti