intro; generalize (isometric_evader t); intro; split;
[ rewrite H; pattern (Rsqr (yp t)) at 1 in |- *;
rewrite <- (Rplus_0_l (Rsqr (yp t))); apply Rplus_le_compat_r;
apply Rle_0_sqr
| rewrite H; pattern (Rsqr (xp t)) at 1 in |- *;
rewrite <- (Rplus_0_r (Rsqr (xp t))); apply Rplus_le_compat_l;
apply Rle_0_sqr ].
Qed.

Lemma YCNGFTYS_evader :
forall t : R, (0 <= t)%R -> (l - v (vi intr) * t <= e t)%R.