<p>
The number of map elements is called its length.
For a map <code>m</code>, it can be discovered using the
built-in function <a href="#Length_and_capacity"><code>len</code></a>
and may change during execution. Elements may be added during execution
using <a href="#Assignment_statements">assignments</a> and retrieved with
<a href="#Index_expressions">index expressions</a>; they may be removed with the

  <mime-type type="application/mbms-msk+xml"/>
  <mime-type type="application/mbms-msk-response+xml"/>
  <mime-type type="application/mbms-protection-description+xml"/>
  <mime-type type="application/mbms-reception-report+xml"/>
  <mime-type type="application/mbms-register+xml"/>
  <mime-type type="application/mbms-register-response+xml"/>
  <mime-type type="application/mbms-user-service-description+xml"/>