x_72"></a> (i.e. the values of
literal expressions) to reside in read-only-memory.  To express this, it is
convenient to imagine that every object that denotes locations is associated
with a flag telling whether that object is mutable<a name="%_idx_74"></a> or
immutable<a name="%_idx_76"></a>.  In such systems literal constants and the strings
returned by <tt>symbol-&gt;string</tt> are immutable objects, while all objects
created by the other procedures listed in this report are mutable.  It is an
error to 