e corresponding <A HREF="unify.html">Trail</A> nodes.
<A NAME="undo_subst_2"></A><HR><PRE><B>void undo_subst_2(<A HREF="unify.html">Trail</A> tr, <A HREF="unify.html">Trail</A> sub_tr);
</B></PRE>It is assumed that <A HREF="unify.html">Trail</A> sub_tr is a subtrail of <A HREF="unify.html">Trail</A> tr.
This routine clears part (maybe all) of a substitution, by
clearing the entries from tr up to, but not including sub_tr.
The corresponding <A HREF="unify.html">Trail</A> nodes are deallocated, so the
caller 