FourLaplacian[exp, p, q] is \frac{\partial}{\partial p_{\mu }}
\frac{\partial}{\partial q_{\mu }} applied to
exp.
Overview, FourDivergence, ThreeDivergence.
SP[q, q]
FourLaplacian[%, q, q]\overline{q}^2
2 D
SOD[q]^OPEmFAD[q, q - p]
FourLaplacian[%, q, q](\Delta \cdot q)^{\text{OPEmFAD}(q,q-p)}
0