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(*                         Some plucker relations                            *)
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Lemma split3b (a b c d e f : point) :
('[a,b,c] * '[d,e,f] =
'[b,c,d] * '[a,e,f] + (-(1)) * '[a,c,d] * '[b,e,f] + '[a,b,d] * '[c,e,f])%f.