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Lemma bracket_free (x : point) k1 (a: point) k2 (b a1 b1: point) :
x = k1 .* a + k2 .* b :> vect ->
'[x, a1, b1] =
(k1 * '[a, a1, b1] + k2 * '[b, a1, b1] )%f.