Upon realizing that : ∂ ∂ ∂ ∂ λ λ ( f ( u 1 , u 2 ) ) = 0 because the dummy variable λ λ cannot appear in the coefficients used to describe the system, we see that the test equation must be true for all sets of constraint equations and thus the system must be holonomic. A similar proof can be conducted with one actual variable in the configuration or state space and two dummy variables to confirm that one-degree-of-freedom systems describable in Pfaffian form are also always holonomic.