Since, in this case, the Legendre Hamiltonian vanishes p ⋅ ⋅ x ˙ ˙ − − L = 0 , the Dirac Hamiltonian is simply the generalized mass constraint (with no interactions it would simply be the ordinary mass shell constraint) H = λ λ [ ( p − − A ) 2 + ( m + S ) 2 ] ≡ ≡ λ λ ( p 2 + m 2 + Φ Φ ( x , p ) ) .