intros Fnum1 Fexp1 H'; replace 0%R with (Fnum1 * 0)%R;
[ apply Rmult_lt_compat_l | ring ]; auto with real zarith.
Qed.

Theorem is_Fzero_rep2 : forall x : float, x = 0%R :>R -> is_Fzero x.
intros x H'.