intro; cut (e T = Die T T)...
intro; rewrite H0 in H; unfold conflict in |- *;
case (Rle_dec (Die T T) ConflictRange); intro...
elim n; assumption...
Qed.

(* Specifications: *)
Lemma L : Rsqr l = (Rsqr (xe - xi intr 0) + Rsqr (ye - yi intr 0))%R.
rewrite xe_0; rewrite ye_0; unfold Rminus in |- *;
rewrite <- (Rplus_comm (- (l * sin beta))); repeat rewrite Rplus_assoc;
repeat rewrite Rplus_opp_r; repeat rewrite Rplus_0_r;
rewrite <- Rsqr_neg; repeat rewrite Rsqr_mult;
rewrite <- (Rmult_comm (Rsqr (sin beta))); rewrite sin2;
unfold Rminus in |- *; rewrite Rmult_plus_distr_r;
ring.