Let's consider a system described by this simple equation:
$x^{2}+y^{2}=5$
this represents a holonomic constraint. Suppose I would like to reduce the degrees of freedom of this system, I should be able to do it since is described by a holonomic constraint. However, both $x^{2}$ and $y^{2}$ are not invertible. So how can I write one of them as a function of the other?