 as it is safer from a typing point of view and
    compatible with concurrent programming. Holes
    should be considered a low-level concept.
  </P>

  <P>See also:
    <A href="future.html"><TT>Future</TT></A>,
    <A href="promise.html"><TT>Promise</TT></A>
  </P>

<?php section("import", "import") ?>

  <P>
    Imported implicitly.
  </P>

<?php section("interface", "interface") ?>

  <PRE>
    signature HOLE =
    sig
	exception Hole

	val hole :     unit -> 'a
	val future :   'a -> 'a

	val fill :   