Corollary

Tag 0DOM, permanent. Cite it as [Bourbaki, Tag 0DOM].

Algebra, chapter IX, § 10. Angles

The right angle (resp. flat angle) is the only element of order 2 of the group $\mathfrak{A}_0$ (resp. $\mathfrak{A}$ ).

Finally, we shall suppose that the plane $E$ is oriented.

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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