Definition Omega (a : R) : bool :=
  match Rle_dec (PI / 2) a with
  | left _ =>
      match Rle_dec a (3 * (PI / 2)) with
      | left _ => true
      | right _ => false
      end
  | right _ => false
  end.

Lemma ye_0 : ye = (yi intr 0 - l * sin beta)%R.