whereupon [ L a ( x → → ) , L b ( y → → ) ] = i f c a b δ δ ( x → → − − y → → ) L c ( x → → ) , [ L a ( x → → ) , R b ( y → → ) ] = 0 , [ R a ( x → → ) , R b ( y → → ) ] = i f c a b δ δ ( x → → − − y → → ) R c ( x → → ) .