A procedure call is interpreted by performing the following steps:
<ul>
<li> Lookup the procedure's definition in the <i>procedure a-list</i>.
<li> Evaluate &#60;expr 1&#62; &#60;expr 2&#62; ... &#60;expr N&#62;
to produce &#60;val 1&#62; &#60;val 2&#62; ... &#60;val N&#62;.
<li> Create a <i>frame</i> consisting of ((&#60;var 1&#62; &#60;val 1&#62;) 
(&#60;var 2&#62; &#60;val 2&#62;)...(&#60;var N&#62; &#60;val N&#62;)).
<li> Push the frame onto the <i>environment stack</i>.
<li> Tail-call the interpreter on a program consisting of:
<ul>
<li> &#60;stmt 1&#62; &#60;stmt 2&#62; ... &#60;stmt M&#62;
<li> A command to pop the environment stack.
<li> The rest of the calling program.
</ul>
</ul>
The environment stack can be popped at the right point by inserting a
special command, e.g., $pop-frame$, into the list of statements to be
interpreted. This should be inserted immediately after &#60;stmt
M&#62; and before the statements comprising the rest of the
program.