ilar to apply_substitute, but the into_term is specified with
a position vector instead of the term itself.  This is so that
the into term can be a variable.  (Recall that variables are
probably shared, and we have to specify an *occurrence*.)
<A NAME="context_to_pairs"></A><HR><PRE><B>Plist context_to_pairs(Ilist varnums, <A HREF="unify.html">Context</A> c);
</B></PRE><A NAME="empty_substitution"></A><HR><PRE><B>BOOL empty_substitution(<A HREF="unify.html">Context</A> s);
</B></PRE><A NAME="fprint_context"