Definition contraction a b k := k *: (b - a) + a.

Lemma upper_dim_dgc1_aux (a p q : 'rV[R]_(2)) (m := (1 / (1 + 1)) *: (p + q)) :
m 0 0 - p 0 0 = 0 -> m 0 1 - p 0 1 != 0 ->
cong a p a q ->
m 0 1 = a 0 1.