Lemma l6_144 : forall A B I A1 A2 B1 B2 C, on_foot A1 B I A -> on_line_d A2 A1 B (0-1) -> on_foot B1 A I B -> on_line_d B2 B1 A (0-1) -> inter_ll C A A2 B B2 -> eq_angle A C I I C B.