Riemann Tensor Notation Explained | Choquet-Bruhat GR

[Maddddd]

Hello
I have been going through the cosmology chapter in Choquet Bruhats GR and Einstein equations and in definition 3.1 of chapter 5 she defines the sectional curvature with a Riemann( X, Y;X, Y) (X and Y two vectors)
I don't understand this notation, regarding the use of the semi colon, is it a derivative or just a way of saying Riemann( X, Y, X, Y)?

 

[martinbn]

No derivative. That's just the Riemann tensor.