Definition row2 {R : ringType} (a b : R) : 'rV[R]_2 :=
\row_p [eta \0 with 0 |-> a, 1 |-> b] p.
Definition a : (@Vector R 1) := row2 0 0.
Definition b : (@Vector R 1) := row2 0 1.
Definition c : (@Vector R 1) := row2 1 0.

Lemma betR_bca : betR b c a = 1.