intuition.
remember (eqb a b) as x.
destruct x.
left.
eapply eqb_leibniz.
auto.
right.
intuition.
subst.
assert (eqb b b = true -> False).
intuition.
congruence.
eapply H0.
eapply eqb_leibniz.
trivial.
Qed.

Theorem dec_EqDec : forall (A : Set)(eqd : eq_dec A),
EqDec A.