One then postulates that for two bodies the Dirac Hamiltonian is the sum of two such mass shell constraints, H i = p i 2 + m i 2 + Φ Φ i ( x 1 , x 2 , p 1 , p 2 ) ≈ ≈ 0 , that is H = λ λ 1 [ p 1 2 + m 1 2 + Φ Φ 1 ( x 1 , x 2 , p 1 , p 2 ) ] + λ λ 2 [ p 2 2 + m 2 2 + Φ Φ 2 ( x 1 , x 2 , p 1 , p 2 ) ] = λ λ 1 H 1 + λ λ 2 H 2 , and that each constraint H i be constant in the proper time associated with H H ˙ ˙ i = { H i , H } ≈ ≈ 0