  When implementing a non-linear constraint in quantum field theory by means of
a Lagrange multiplier, $\l(x)$, it is often the case that quantum dynamics
induce quadratic and even higher order terms in $\l(x)$, which then does not
enforce the constraint anymore. This is illustrated in the case of Unimodular
Gravity, where the constraint is that the metric tensor has to be unimodular
($g(x)\equiv \det\,g_{\m\n}(x)=-1$).
