A mounth ago, I noticed a command which could express an expression by a charactor. For example: We can define r equals sqrt(x^2+y^2+z^2). When we compute ∂_xr, it will give the answer like x/r. In the examples, this compared with 'subs','eval','algsubs'. So I am sure it is different from the three commands before. Now I need to use this command, but I cannot remembered what it is. Who can tell me. Thanks.