ve
tail calls.  A call is <em>active</em> if the called procedure may still
return.  Note that this includes calls that may be returned from either
by the current continuation or by continuations captured earlier by
<tt>call-with-current-continuation</tt> that are later invoked.
In the absence of captured continuations, calls could
return at most once and the active calls would be those that had not
yet returned.
A formal definition of proper tail recursion can be found
in&nbsp;[<a href="r5rs-Z-H-14.html#ci