HREF="#unify_bt_next">unify_bt_next</A>(bt);
      }
    free_context(c1);
    free_context(c2);
  }
</PRE>
The routine <A HREF="#unify_bt_next">unify_bt_next</A>() takes care of clearing the substitutions
before getting the next unifier.
If you decide not to get all of the unifiers, you should call
<A HREF="#unify_bt_cancel">unify_bt_cancel</A>() to free the memory used by the <A HREF="btu.html">Btu_state</A>.
<P>
If there are no AC or C symbols, it is a little bit faster to
use unify() (ordinary unificati