Lemma l6_56 : forall A B C E G M, is_midpoint E A C -> on_line_d G B E (2/(2+1)) -> 1+2 <> 0 -> (2+1) * Py M G M + Py A G A + Py G C G + Py B G B = Py A M A + Py B M B + Py C M C.