ify</A>(),
<A HREF="#apply">apply</A>(), and <A HREF="#undo_subst">undo_subst</A>().  Assume we have terms t1 and t2.
(Terms t1 and t2 may share variables, but we "separate" the
variables by using different contexts.  That is, variable v1 in
context c1 is different from variable v1 in context c2.)
<PRE>
    {
        <A HREF="unify.html">Context</A> c1 = <A HREF="#get_context">get_context</A>();
        <A HREF="unify.html">Context</A> c2 = <A HREF="#get_context">get_context</A>();
        <A HREF="unify.ht