Good point. I should have specified that I am only interested in autonomous systems. It is clear now that my initial post lacked some detail which is probably important. I need to derive the equations of motion for a system symbolically, e.g. via Lagrangian mechanics, and solve the resulting symbolic equations so that I can then use the tools in DifferentialEquations, BoundaryValueDiffEq, etc., to solve numerical problems. The systems I am working with have ODEs which are rather unpleasant to look at, so I am looking for a way to make computing the equations of motion autonomous so that the output is a system of the form x'(t) = f(x(t)). The input, I suppose, would be a symbolic expression (a Lagrangian).