adjoint

f xlist adjoint g
poly f; string xlist; poly g;
g is the adjoint operator of f.
The variables to take adjoint are specified by xlist.
Example: [(x,y) ring_of_differential_operators 0] define_ring
          (x^2 Dx - y x Dx Dy-2). (x,y) adjoint 
          ((-Dx) x^2 - (-Dx) (-Dy) x y -2). dehomogenize sub :: ==> 0