The canonical momentum is p = ∂ ∂ L ∂ ∂ x ˙ ˙ = ( m + S ( x ) ) x ˙ ˙ − − x ˙ ˙ 2 + A ( x ) and by squaring leads to the generalized mass shell condition or generalized mass shell constraint ( p − − A ) 2 + ( m + S ) 2 = 0.