Hello, I was set the problem to differentiate the following streamfunction in order to find its x and y velocities: psi=psinought*tanh(y/d)+mu*exp(-((x-L)^2+y^2)/2*sigma^2)*cos(k(y-vt)). Being the lazy sort I am, I called in Maple. dpsi/dx worked out perfectly (in that I got the same answer by hand) dpsi/dy, however, threw up the following: dpsi/dy=(psinought/d)*(1-tanh(y/d)^2)-(mu*y/sigma^2)*exp(...)*cos(...)-mu*exp(...)*sin(...)*D(k)(y-vt) the last term D(k)... makes no sense to me. How can differentiating introduce a new variable? I know I'm probably being a moron, and I'm certainly no expert in using maple, but anyone who can shine a light in my general direction would be muchly appreciated.