Definition eval_beq a b :=
match a,b with
| feval_cast t1_1 t2_1 , feval_cast t1_2 t2_2 => andb (expr.eqb_type t1_1 t1_2)
                                                      (expr.eqb_type t2_1 t2_2)
| fderef_noload t1, fderef_noload t2 => expr.eqb_type t1 t2
| feval_field t1 id1 , feval_field t2 id2 =>  andb (expr.eqb_type t1 t2)
                                                   (BinPos.Pos.eqb id1 id2)
| feval_binop op1 t1_1 t2_1, feval_binop op2 t1_2 t2_2 =>
              andb (andb (binary_op_beq op1 op2)
                   (expr.eqb_type t1_1 t1_2))
                   (expr.eqb_type t2_1 t2_2)
| feval_unop op1 t1 , feval_unop op2 t2 =>
              andb (unary_op_beq op1 op2)
                   (expr.eqb_type t1 t2)
| feval_id id1, feval_id id2 => BinPos.Pos.eqb id1 id2
| _, _ => false
end.

Lemma eval_beq_sound : forall a b, eval_beq a b = true -> a = b.