<?php include("macros.php3"); ?>
<?php heading("The Hole structure", "The <TT>Hole</TT> structure") ?>

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

  <PRE>
    signature HOLE
    structure Hole : HOLE
  </PRE>

  <P>
    The <TT>Hole</TT> structure provides operations to create and inspect
    <EM>holes</EM>. A hole is a place-holder for an undetermined value,
    reminiscent of a "logic" variable in logic programming. A hole is very
    much like a <A href="promise.html"><I>promise</I></A>, but is transparen