Hey,
when calculating the Riemann curvature tensor, you need to calculate the commutator of some vector field V , ie like this :-
[\bigtriangledown_a, \bigtriangledown_b] = \bigtriangledown_a\bigtriangledown_b - \bigtriangledown_b\bigtriangledown_a = V;_a_b - V;_b_a
But...