ring_of_differential_operators

string ring_of_differential_operators
This command is used to define a ring of differential operators 
with the macros define_ring, weight_vector and elimination_order.
The user cannot use the variable names h, e, H and E.
cf. show_ring, system_variable --- CurrentRing
    print_switch_status, switch_function
Example: [(x,y) ring_of_differential_operators [[(Dx) 1 (Dy) 1]]
           weight_vector 0] define_ring
   Define the ring of differential operators Q<x,y,Dx,Dy> with the order
   obtained by refining the partial order defined by the weight vector
   [x,y,Dx,Dy] = [0,0,1,1] 
Example: [(x,y) ring_of_differential_operators 0 [(weightedHomogenization) 1]]
          define_ring 
Example: [(x,y) ring_of_differential_operators [[(x) -1 (Dx) 1]] weight_vector
          0 [(degreeShift) [[1 0 1]]] ]
          define_ring 
Example: [(x,y,z) ring_of_differential_operators (Dx,Dy) elimination_order 0] define_ring
         Dx,Dy, and Dz are corresponding differential operators to the 
         space variables x, y and z.