string ring_of_difference_operators !!! OBSOLETE !!! This command has not been maintained since 1996. So, there may be troubles if it is used with new features of kan/sm1. This command is used to define a ring of difference 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_difference_operators (Ex,Ey) elimination_order 0] define_ring Ex,Ey, and Ez are corresponding difference operators to the space variables x, y and z ( Ex x = (x+1) Ex).