Fetch the current delay list. If this list is not empty, the
current answer is undefined. In the logical sense, this
function always succeeds and sets dl to the delay list. It
returns
FALSE if the delay list is empty (and answer is well
defined) and TRUE if the delay list is not empty. If dl
is 0 no list is instantiated, while the return value is the same. This
allows for testing that an answer is undefined as below.
if ( PL_get_delay_list(0) )
<undefined>
else
<normal answer>
For now, we consider the content of the list elements opaque. See
boot/tabling.pl for examples.
