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,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.