_{1},x

_{2},...,x

_{n}}. If x

_{1}= f(x

_{2},x

_{3},...,x

_{n}) then this represents a constraint on all the variables. In this case, it's possible to find dx

_{1}/dx

_{i}as long as f is differentiable. But not all possible constraints among the x

_{j}are of this form. How might one extend the concept of derivative to the case when the constraint on all the variables is not necessarily that one variable is a function of the others?