| @(expr,I,...,N) |
The operator of partial derivation is denoted by @ and its arguments are as follows: the first is the expression which will be differentiated and the others are the "variables" of the differentiation. The "variables" of the differentiation are assumed to be abstract or concrete indices and their number gives the rank of the derivative. If metric tensor has previously been introduced then the derivation is possible with respect to upper indices too, but they are converted automatically to lower indices by proper metric tensors.
