0A0;EmptyStk
</div>
     <p class="noindent"></div>
<p class="noindent"> Modules importing <span 
class="pcrr7t-">Stack</span> cannot construct values of type <span 
class="pcrr7t-">StkType</span> because they do not have access to the
constructors of the type. Instead, they must use <span 
class="pcrr7t-">push</span>, <span 
class="pcrr7t-">pop</span>, and <span 
class="pcrr7t-">empty</span> to construct such values.
 <p class="noindent"> It is also possible to build an ADT on top of an existing type by us