html">Context</A> c2);
</B></PRE>This routine gets the first unifier for a pair of terms and
returns a <A HREF="btu.html">Btu_state</A> (or NULL if there are no unifiers) to be
used for calls to <A HREF="#unify_bt_next">unify_bt_next</A>() to get the rest of the unifiers.
This unification handles associative-commutative (AC) and 
commutative (C) symbols, so there can be more than one unifier.
(Commutatvie unification is primitive, and you can get duplicate unifiers.)
<P>
This is called "backtrack unificatio