I have seen that the Mathematica product can do this, but I've yet to figure out how to indicate to W|A that variables are dependent, rather than independent. I know that the product can do this, because equations such as finding d/dx of $$xe^x=ye^x$$ produce correct answers ... following along in the solutions. But if I take this result, properly reduced, d/dx of $$\frac{y(x-1)}{x(y-1)}$$ it assumes that the variables are independent. I've tried all sorts of notation without finding a method to instruct to W|A that x and y are dependent variables. Thank you for any answers to my question. BTW I am a paying member of W|A. :-)