I added a comment to the test case  # Exact differential => d(V(P, T)) => dV/dP DP + dV/dT DT 

This is the Wester test_U7  description  in Macsyma:
(c27) /* Exact differential => d(V(P, T)) => dV/dP DP + dV/dT DT */
diff(V(P, T));
Time= 0 msecs
		  d			  d
(d27) 		 (-- (v(p, t))) del(t) + (-- (v(p, t))) del(p)
		  dt			  dp